Thanks for the info. I went through with it and nothing bad happened. However, the issue still remains.
The thing is, I haven't changed any settings that would cause this problem to occur. In the "set which pictures are displayed", I previously had only "Embedded". I now added "Cover.*" as well and rescanned the artwork but some genre categories still shows the "no album art" picture in the thumbnail browser. I've tried browsing by a couple of other tags, custom tags and virtual tags and it would seem that genre category is the only one affected by this.
I've tried to restart MB, use a different tab, all with the same result as above.
That's already done. No auto-retrieval of artwork.
Edit: It's worth mentioning that this problem only occurs if no filter is selected. If I choose the filter "Soundtrack", the thumbnail browser displays a thumbnail for the genre category "Comedy/Spoken" which it will not do if no filter is selected.