The db should have stayed in a consistent image (probably with half images registered). That's the db server duty, after all.
But then the oddities started, sometimes pictures that I uploaded (no matter whether via imageImport or one by one with the normal web-uploader) started to randomely disappear and re-appear. Unfortunately these problems prevailed, so I decided to look into the problem a bit.
The strange thing is, e.g. looking at Special:NewFiles, the thumbnails and Image-Info are sometimes there, but doing Refresh in the browser makes them alternatingly go away and re-appear. (I tried with both Firefox and Chromium.) When looking at file-links it's the same, sometimes they're red, then they're blue.
This doesn't make sense. You mentioned that you cleared cache, what about memcached? (objectcache table if not using memcached).
So all in all I'm kinda assuming that this is a database-issue, but i have no clue how to "reset" the database without having to restore my content by hand (which I'd really rather not do, because it's a lot of work).
It seems a db problem. Have you tried to eg. stop & restart the db server? Which engine are you using? What about running importImages *again*? (so that half-updated files get added). You could also remove everything from image table, and readd with importImages (well, you would lost the information on author and upload time, although could be consulted through Special:Log).
Hey, thanks for your suggestions, I think I figured it out now (even though I cannot clearly tell you what the real root of the problem was). I followed your last piece of advice and ran importImages again on all files (I first dropped everything uploaded by the maintenance script to clear the table) and now the issue seems to be gone. In case this pops up again, I'll take a closer look at the db. I'm not entirely sure I can really start/stop the db-server (MySQL5) since my only access to it is phpmyadmin.
Many thanks, Simon