Temporarily solved by putting in the Items setting in the SharedStrings.resx but since the error occured im thinking something did not went ok with the upgrade. One thing I noticed was that up in the right corner in Commerce Manager it still says Version 5.2 build 377 where I would suspect it would say something like build 628 since the new Mediachase.Commerce.dll version is 5.2.628.0.

Yeah, it should say 628 up in the corner as well. Try doing the upgrade once more and look at the output to see if there is any errors present in the log.

Tried the upgrade again, no errors but no difference, still build 377. Just to be clear, the site to be upgraded is the regular CMS site and not the manager site, right? The only site visible in the list when I first upgraded was the CMS site and looking in the Managersite it got some new dlls and things like that so it seems that everything worked.

Yes, uppgrading the CMS site will upgrade the Commerce Manager as well. It should also update teh eCommerceFramework folder for that specifc Commerce Manager. 

So maybe Commerce Manager has been upgraded, but not the right eCommerceFramework folder?

When looking at this I found one error related to the Commerce bin folder config setting which pointed to another folder than the one used by the site which expllained the missing resource file. Changed this and ran the upgrade procedure again. No resource eroor now but a bunch of other errors and still version 377.

When editing a product the error
CS0117: 'ASP.apps_catalog_tabs_entryoverviewedittab_ascx' does not contain a definition for 'AvailableFromValidator_Validate'
occurs referring to the eCommerceFramework folder Shared\Apps\Catalog\Tabs\EntryOverviewEditTab.ascx

Examining this folder I can see from the versioning control that these files have been updated so the eCommerceFramework folder gets updated correctly as it would seem.

Another error that occurs is when accessing a MetaField. Then the classic object reference not set as an instance of an object occurs with this stacktrace:

 Mediachase.Commerce.Manager.Core.MetaData.Admin.MetaFieldEditControl.InitDataTypeUI(MetaDataType type) +135
   Mediachase.Commerce.Manager.Core.MetaData.Admin.MetaFieldEditControl.BindData() +99
   Mediachase.Commerce.Manager.Core.MetaData.Admin.MetaFieldEditControl.Page_Load(Object sender, EventArgs e) +490
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +24
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +41
   System.Web.UI.Control.OnLoad(EventArgs e) +131
   System.Web.UI.Control.LoadRecursive() +65
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Control.LoadRecursive() +190
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2427

One more error occurs when accessing the Import under Business Foundation in the /Apps/Core/Controls/FileListControl.ascx

Could not load file or assembly 'ComponentArt.Web.UI, Version=2008.1.1202.35, Culture=neutral, PublicKeyToken=41d2e7a615ba286c' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

The reason for the upgrade was to make some of our code a bit more effective as recommended by EPiServer support but it didn't seem so simple to perform the upgrade.

Solved the issue. Somehow RedGate .Netreflector had blocked som mediachase commerce files preventing a correct update of the files.