From the error my guess would be that there is a link somewhere in content that is stored with a slash after language. That is somewhere in content in db there would be a link something like /link/<guid>.aspx?epslanguage=da/

You could try to run following queries from SQL Managment Studio 

select LongString from tblContentProperty where LongString like '%epslanguage=da/%'
select String from tblContentProperty where String like '%epslanguage=da/%'

I tried your queries with no result.

I narrowed the errors down to two internal shortcuts between to pages and one of their subpages. In the EPiServerErrorLog the exceptions always occur when trying to load the ContentIDs of one of the two shortcut destinations.

For some reason if I disable or redirect the shortcuts the site works.

Another temporary workaroung I found, is that if I navigate to one of the shortcut destinations in edit mode, the whole site render again (both in edit mode and in view mode). Or I can navigate to the second of the shortcuts in view mode, and the whole site works, too.
Then when the IIS application restarts, the errors are back again.

This is an extract of the error log, when the IIS was restarted and the error occurred:

2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.ContentProvider+<>c__DisplayClass1c.<LoadContentFromCacheOrRepository>b__1a - ContentLink=3640, ReadAndCacheObject delegate...LoadContent returned Content object
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EPPageData:3640, ReadAndCacheObject delegate returned object of type TObject
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.ContentProvider.LoadContentFromCacheOrRepository - ContentLink=3640, returning ContentReference object
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EP:LanguageBranch
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EP:LanguageBranch
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EP:LanguageBranch, retry 1, calling CacheManager.Get for key
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EP:LanguageBranch, retry 1, CacheManager.Get returned object of type EPiServer.DataAbstraction.LanguageBranchCollection[] for key
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EP:LanguageBranch, retry 1, returning object of type TObject
2014-01-08 16:22:02,448 INFO [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EP:LanguageBranch, returning an object of type TObject
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EPPageData:3651:da/
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EPPageData:3651:da/
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EPPageData:3651:da/, retry 1, calling CacheManager.Get for key
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EPPageData:3651:da/, retry 1, CacheManager.Get returned null for key
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.TryRead - cacheKey=EPPageData:3651:da/, retry 1, returning null
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EPPageData:3651:da/, inserting a RefCountedEvent object in cache
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EPPageData:3651:da/, calling ReadAndCacheObject delegate passed
2014-01-08 16:22:02,449 INFO [42] EPiServer.Core.ContentProvider+<>c__DisplayClass1c.<LoadContentFromCacheOrRepository>b__1a - ContentLink=3651, ReadAndCacheObject delegate...calling LoadContent
2014-01-08 16:22:02,449 ERROR [42] EPiServer.Core.OptimisticCache`1.Read - cacheKey=EPPageData:3651:da/, last exception was:
System.Globalization.CultureNotFoundException: Culture is not supported.
Parameter name: name
da/ is an invalid culture identifier.
   at System.Globalization.CultureInfo.GetCultureInfo(String name)
   at EPiServer.Core.LanguageSelectorContext.get_SelectedLanguageBranch()
   at EPiServer.DefaultContentProvider.LoadContent(ContentReference contentLink, ILanguageSelector languageSelector)
   at EPiServer.Core.ContentProvider.<>c__DisplayClass1c.<LoadContentFromCacheOrRepository>b__1a()
   at EPiServer.Core.OptimisticCache`1.Read(String cacheKey, ReadAndCacheObject`1 readAndCacheObject)
2014-01-08 16:22:18,461 ERROR [42] EPiServer.Global.Global_Error - 1.2.5 Unhandled exception in ASP.NET
System.Globalization.CultureNotFoundException: Culture is not supported.
Parameter name: name
da/ is an invalid culture identifier.
   at System.Globalization.CultureInfo.GetCultureInfo(String name)
   at EPiServer.Core.LanguageSelectorContext.get_SelectedLanguageBranch()
   at EPiServer.DefaultContentProvider.LoadContent(ContentReference contentLink, ILanguageSelector languageSelector)
   at EPiServer.Core.ContentProvider.<>c__DisplayClass1c.<LoadContentFromCacheOrRepository>b__1a()
   at EPiServer.Core.OptimisticCache`1.Read(String cacheKey, ReadAndCacheObject`1 readAndCacheObject)
   at EPiServer.Core.ContentProvider.LoadContentFromCacheOrRepository(ContentReference contentreference, ILanguageSelector selector)
   at EPiServer.Core.ContentProvider.Load(ContentReference contentLink, ILanguageSelector selector)
   at EPiServer.DataFactory.TryGet[T](ContentReference contentLink, ILanguageSelector languageSelector, T& content)
   at EPiServer.DataFactory.Get[T](ContentReference contentLink, ILanguageSelector languageSelector)
   at EPiServer.Web.Routing.UrlResolver.GetVirtualPath(ContentReference contentLink, String language, ContextMode contextMode, RouteValueDictionary routeValues, RequestContext requestContext)
   at EPiServer.Web.Routing.UrlResolver.GetVirtualPath(ContentReference contentLink, String language, VirtualPathArguments virtualPathArguments)
   at EPiServer.Web.Routing.UrlResolver.GetUrl(ContentReference contentLink, String language)
   at EPiServer.Web.HierarchicalUrlRewriteProvider.GetFriendlyPagePath(ContentReference contentReference, String currentLanguageBranch)
   at EPiServer.Web.HierarchicalUrlRewriteProvider.ConvertToExternalInternal(UrlBuilder url, Object internalObject, Encoding toEncoding)
   at EPiServer.Web.HierarchicalUrlRewriteProvider.ConvertToExternal(UrlBuilder url, Object internalObject, Encoding toEncoding)
   at EPiServer.Web.FriendlyHtmlRewriteToExternal.HtmlRewriteUrl(UrlBuilder internalUrl, UrlBuilder externalUrl, UrlBuilder url, Encoding encoding, RebaseAction rebase, Object& internalObject)
   at EPiServer.Web.FriendlyHtmlRewriteToExternal.rewritePipe_HtmlRewriteUrl(Object sender, HtmlRewriteEventArgs e)
   at EPiServer.Web.RewritePipe.OnHtmlRewriteUrl(HtmlRewriteEventArgs e)
   at EPiServer.Web.HtmlRewritePipe.WriteElement(MyHtmlRewriteEventArgs e, SgmlReader reader, TextWriter writer)
   at EPiServer.Web.HtmlRewritePipe.RewriteToEnd(TextReader reader, TextWriter writer)
   at EPiServer.Web.HtmlRewriteStream.Close()
   at System.Web.HttpWriter.FilterIntegrated(Boolean finalFiltering, IIS7WorkerRequest wr)
   at System.Web.HttpResponse.FilterOutput()
   at System.Web.HttpApplication.CallFilterExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

I have the same problem after upgrading from CMS 7.1 -> 7.5 

[CultureNotFoundException: Culture is not supported.
Parameter name: name
sv~ is an invalid culture identifier.]
System.Globalization.CultureInfo.GetCultureInfo(String name) +14293038
EPiServer.Core.LanguageSelectorContext.get_SelectedLanguageBranch() +40
EPiServer.DefaultContentProvider.LoadContent(ContentReference contentLink, ILanguageSelector languageSelector) +221
EPiServer.Core.&lt;&gt;c__DisplayClass1c.&lt;LoadContentFromCacheOrRepository&gt;b__1a() +141
EPiServer.Core.OptimisticCache`1.Read(String cacheKey, ReadAndCacheObject`1 readAndCacheObject) +1175
EPiServer.Core.ContentProvider.LoadContentFromCacheOrRepository(ContentReference contentreference, ILanguageSelector selector) +945
EPiServer.Core.ContentProvider.Load(ContentReference contentLink, ILanguageSelector selector) +233
EPiServer.DataFactory.TryGet(ContentReference contentLink, ILanguageSelector languageSelector, T&amp; content) +526
EPiServer.DataFactory.Get(ContentReference contentLink, ILanguageSelector languageSelector) +86
EPiServer.Web.Routing.UrlResolver.GetVirtualPath(ContentReference contentLink, String language, ContextMode contextMode, RouteValueDictionary routeValues, RequestContext requestContext) +93
EPiServer.Web.Routing.UrlResolver.GetVirtualPath(ContentReference contentLink, String language, VirtualPathArguments virtualPathArguments) +130
EPiServer.Web.Routing.UrlResolver.GetUrl(ContentReference contentLink, String language) +18
EPiServer.Web.HierarchicalUrlRewriteProvider.ConvertToExternalInternal(UrlBuilder url, Object internalObject, Encoding toEncoding) +262
EPiServer.Web.HierarchicalUrlRewriteProvider.ConvertToExternal(UrlBuilder url, Object internalObject, Encoding toEncoding) +240
EPiServer.Web.FriendlyHtmlRewriteToExternal.HtmlRewriteUrl(UrlBuilder internalUrl, UrlBuilder externalUrl, UrlBuilder url, Encoding encoding, RebaseAction rebase, Object&amp; internalObject) +230
EPiServer.Web.FriendlyHtmlRewriteToExternal.rewritePipe_HtmlRewriteUrl(Object sender, HtmlRewriteEventArgs e) +494
EPiServer.Web.HtmlRewritePipe.WriteElement(MyHtmlRewriteEventArgs e, SgmlReader reader, TextWriter writer) +575
EPiServer.Web.HtmlRewritePipe.RewriteToEnd(TextReader reader, TextWriter writer) +1306
EPiServer.Web.HtmlRewriteStream.Close() +220
Microsoft.VisualStudio.Web.PageInspector.Runtime.Tracing.ArteryFilter.Close() +46
System.Web.HttpWriter.FilterIntegrated(Boolean finalFiltering, IIS7WorkerRequest wr) +12599767
System.Web.HttpResponse.FilterOutput() +121
System.Web.CallFilterExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +119
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously) +165

I have searched for the string:  sv~ in the entire DB but no luck.

The site started once but efter editing License.config the error occured. One more strange thing the url ...util/login.aspx generates in 404 page not found this is probably from the above error maybe something about redirect.

What caused the issue in my project, was that one of the user controls was building URL's (for a menu) by appending a slash to the PageData.LinkURL.
And since LinkURL is formatted like: PageTemplate.aspx?id=100&epslanguage=da, the trailing slash was messing up the parsing of the QueryString.

I ended up deleting the slash.
Maybe this will help you out.

Well I have not identified the problem yet, but as it seems its only the startpage that generates the error, not any subpages, I have tried removing calls to blocks and usercontrols, but no light in the tunnel yet.

Another strange issue is that I'm getting 404 in the /util/login.aspx page.

The upgrade of the site missed the appdata folder which was not upgraded to the latest version, I compared to a default installation of Alloy and updated the sites appdata folder and now I can login as expected. 

The culture issue has its origin in one of the startpage's blocks, on the startpage there is 5 blocks in a ContentArea and one of these causing the problem?