Ok, I'll try that again - I tried before but I had not seen the reference to "two folders" when I tried. Which two are they (I can look it up in the other thread I guess)?
Thanks for the tracker-processes tip. I have similar issues with album art. Changing covers in mp3 files didn't change them in Media Player. I've found that the thumbnails are stored in .thumbnails/cropped and the covers themself in .cache/media-art. Both of them in /home/user of course. So I deleted the entire directories and... worked! The Media Player now shows the new album covers.