This is really strange.
The issue is there even with the default Protostar template.
I have no idea what's causing the issue, but it does seem to be a cache related problem.
I tried to delete the cache from the Joomla Admin... it says that the cache was deleted but the cache items are still shown on the screen.
I really have not idea what's going on. It is either some server-side caching, a permissions issue (you do not have enough permissions to delete the cache on the server) or it is a corrupted Joomla installation.
Sorry Marcel, I just have no idea and cannot help you further