Thread:Project:Support desk/MediaWiki:Skinname.css stopped working

On our wiki skin specific styling via MediaWiki:Skinname.css stopped working. This applies for custom skins as well as for Vector & Co. Changes to the css files in the file system still take effect as well as changes to Common.css. Tested on various recent versions of FF & IE.

The issue occured on MW 1.19.0 with actually no changes being made that could have caused it. Meanwhile we've upgraded to MW 1.20.3 and the issues persist. On a similar install everything is still working as expected.

Any ideas?

Eric