Upgrade from 12 to 13 Issue

I currently have the following situation: We mostly finished the upgrade from 12 to 13 and deployed the new version on our test system.
Locally everything works fine but on our server suddenly all assets on the skin servlet path /skin return 404. This only happens for that path. When the path is accessed locally from within the server the problem also doesn’t occur.

Does anybody have a hinch what the problem could be? Could be a problem with the hosts configuration?

Is this custom skin css like in [1]? Then check the wicket.skin property editor:templates/default/root

[1] Tweak the CSS of the CMS Editing Templates - Bloomreach Experience - Headless Digital Experience Platform