Since you have default values for all custom segments {page}, {tags}, {query}, {sortorder} then will that custom route handle request that does not contain those segments (using the default values). If you make some segment mandatory then it will not  interfer when that value is not present.

Another option if that route is only to be used form incoming route (that is that you never want the UrlResolver to create such outgoing links)  then you can set Direction on MapContentRouteParameters to SupportedDirection.Incoming

If none of the above is feasible then you could try to move the route registration so it gets called after the route serving site assets.

I´m not entirely sure on how to make some segments mandatory as you suggested, custom routing isn´t something that I´ve been working with that much :) I set the parameters to having a default value (query = "" etc) instead of UrlParameter.Optional as there was no UrlParameter.Mandatory, but that didn´t do any difference, it still messed upp the SiteAssets URL to SysSiteAssets. The SupportedDirection.Incoming however seemed to do the trick actually.

But please enlighten me how to set the parameters to be mandatory if I´m supposed to do it in any different way so I can test it correctly.

Thanks!

Just remove the entry from the default object. So for example

var defaults = new { controller = "ListPage", page = 1, query = UrlParameter.Optional, tags = UrlParameter.Optional, sortorder = UrlParameter.Optional, action = "index" };

specifies default values for controller, page, query, tags, sortorder and action. So for example remove the tags part above so you get
var defaults = new { controller = "ListPage", page = 1, query = UrlParameter.Optional, sortorder = UrlParameter.Optional, action = "index" };

then in your pattern "{language}/{node}/{page}/{tags}/{query}/{sortorder}" it means tags must be part of the url to match on incoming routing. And for outgoing url generation it means "tags" must be part of route values.

Putting route last in chain is usually a great idea to avoid breaking default routing of episerver. Highly recommended :)

Noted Daniel :) I know the order matters when you have several custom routes (first match first serve), but can I "force" my custom route(s) to be last in chain somehow?

I removed the parameters so it looked like this: 

var defaults = new { controller = "ListPage", action = "index" };

and the pattern looked like this:

routes.MapContentRoute("defaultRoute", "{language}/{node}", defaults, defaultMapContentRouteParameters);

But that made the whole custom route not work at all. I also left the page parameter and the page part in the pattern but that didn´t have any affect either.

So what actually worked is the SupportedDirection which is what we might go with as the UrlResolver doesn´t need to be able to create URLs with the parameters used.

 [ModuleDependency(typeof(EPiServer.Web.InitializationModule), typeof(StructureMapInitialization))]
    public class CustomRoutesModule : IConfigurableModule
    {
        public void Initialize(InitializationEngine context)
        {
            EPiServer.Global.RoutesRegistered += Global_RoutesRegistered;
        }
        void Global_RoutesRegistered(object sender, RouteRegistrationEventArgs e)
        {
            e.Routes.Add("custom301s", new SeoManagerRoute());
        }
   }

You can use the RoutesRegisteredEvent like above to add custom routes after the Episerver ones...I use it to add 301s for pages that Episerver can't route for instance.