Folder icons missing

[Chesta 0]

I've had Emby for quite some time now and everything worked fine. But some weeks ago, the icons of my "Movies" and "Shows" folders just disappeared. There now are empty placeholders.
It looks like this:

I can also edit them, where I can see there is a placeholder instead of the preview/thumbnail:

 

I tried removing them and manually scanning the library again, but the only thing this did was it re-enabled these placeholders.

It used to work perfectly, what could have caused this issue? I don't have any other issues using Emby, and this looks like the sort of issue that can be easily solved with the right know-how.

I tried removing them and manually scanning the library again, but the only thing this did was it re-enabled these placeholders.