Not for the first time after an update (on Kubuntu), QuiteRSS has apparently overwritten my feeds.db, replacing it with some standard that only contains the QuiteRSS-feed. What the hell are you doing with your updates? That should not ever happen!
Now I have to look for an earlier backup, most probably with many of my unread news gone. Thank you very much. I think I will stop using QuiteRSS, as this is just stupid and extremely annoying.