I think I may be having a similiar problem... I've been searching through the threads and this is the closet to my issue, anyway, of recent topics:
After upgrading to the 11.1.73 Beta, most of my cover art stopped displaying in the Playing Now Display schemes (such as Cover Art and Track Info - DJ2).... and the thumbnails for these albums are no longer there in the album cover thumbnail view, either. I can't blame 11.1.73 specifically since the problem may have happened with an earlier beta, but I just noticed after this upgrade, so it seems to be the culprit.
I had my art stored as folder.jpg in each album folder previously, but with MC 11.1.73 Beta I'd selected the "Next to File ([Artist] - [Album]).jpg)" option.... and I suspect this had something to do with what's gone wrong: choosing this option didn't seem to update my old folder.jpg images but it did work for new cover images -- and it's now only the ([Artist] - [Album]).jpg files that are generating thumbnails and getting displayed in MC. I think. I'm not 100% positive about how this is breaking down; I'm just sure that a few thousand album covers are no longer displaying altough the art is still lcoated in the folders.
So I tried renaming all of the olf folder.jpg images to this new naming convention, but it didn't help. I tried doing a "Quick Find Cover Art" for all of my files, and it seems like MC is finding the files -- but then most of them still don't get displayed.
More interesting still: For the Display view called "Track Info - DJ2", if no cover art is available at all then the MC logo is displayed, but if the cover art IS in the album folder but isn't displaying (because it was put in the album folder prior to the MC 11.1.73 upgrade?) then I get a little white box with a red X in it instead of the image.... which is now the giveaway that I do have image art available for the album, but it's just not displaying, evne though it's perfectly good JPG that was working with previous versions of MC.
Maybe I'm looking at the situation wrong or MC is actually trying to update my image files in the background and I should just give it time? I really dunno what's going on.... I'll try upgrading to MC 11.1.74 and I'll try renaming ALL images back to folder.jpg next and see if that works.
But if anyone has an idea as to why this has gone haywire for me, shout it out....
thanks,
Luke