Hey! I have same issue. First image was uploaded yesterday (thumbnail gone blank after while, maybe exactly 30 minutes like Constantine Art wrote). Second image was uploaded 3 days ago and thumbnail looks ok.
It appears to have been a bug that hopefully has since been fixed. I didn't even have to wait 30 minutes. 3 out 4 images has broken thumbnails. I deleted them and re-loaded and they worked fine.
Hint: Before you delete the broken ones, copy and paste your titles and keywords into a notepad and then reupload.
Adobe Community Expert | If you can't fix it, hide it; if you can't hide it, delete it.
Not sure how a firewall can meticulously erase your thumbnails in the review queue exactly 30 minutes after submitting them but no. This was not an issue at all till yesterday when they also introduced this "challenge" to select all cats from a grid of 9 images to verify you're human. Probably not related but just saying something changed and now we have this "bug"....
Having the same issue as of this evening. I just do my normal uploads, put them in the review queue and after approx 30 minutes the thumbnails go blank.