Frontend errors don't occur. The browser error console is not showing anything. Also tried IE and Firefox just to see if the problem is browser based. But nothing.

The DNN Event Viewer shows an exception:AssemblyVersion:7.3.2

PortalID:0
PortalName:e-Wolf
UserID:4405
UserName:Sebastian_Beyer@my-company.com
ActiveTabID:63
ActiveTabName:Skins
RawURL:/de-de/admin/skins.aspx
AbsoluteURL:/Default.aspx
AbsoluteURLReferrer:http://localhost:8080/de-de/admin/skins.aspx
UserAgent:Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
DefaultDataProvider:DotNetNuke.Data.SqlDataProvider, DotNetNuke
ExceptionGUID:06a3ffa8-c035-48cc-9699-215e9b8e6688
InnerException:Ein Element mit dem gleichen Schlüssel wurde bereits hinzugefügt.
FileName:
FileLineNumber:0
FileColumnNumber:0
Method:System.Collections.Generic.Dictionary`2.Insert
StackTrace:
Message:
System.ArgumentException: Ein Element mit dem gleichen Schlüssel wurde bereits hinzugefügt.
bei System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
bei DotNetNuke.Entities.Portals.PortalController.GetPortalSettingsDictionaryCallback(CacheItemArgs cacheItemArgs)
Source:
Server-Name: <MYCOMPUTERNAME>

Does this help?

And one more question. Where in the database is this information stored? Or is the skin also set in the user-settings, cookies somethere else?

Kind regards

Thanks a lot. That fixed the issue.

By the way: Usability in this forum has two main problems. The quick reply feature doesn't indicate if the reply has been sent and the editor to reply in a new window  constantly keeps crashing Chrome and doesn't show the last thread item you're replying to. Instead it show your initial question. But I think there is already a thread for that.