I updated all tables specified, checked the frontend. Everything looked fine, except for missing thumbnails.
So I went straight to the backend, checked the configuration, all fine, and ran "Check for and create missing thumbnails."
And it gave me a lot like this:
"Checking photo #10821: image /var/www/poisonfree/community/gallery/data/500/816894564041_m.gif not found, removing from database... done."
Before: over 1GB of photos
After: 24 MB
I was close to a heartattack until I figured out that /data still holds 1.5 GB of data, but what happened?!?
Pooohaaaa - Imagine me, one day before a short holiday trip to the Czech Republic, 2:00 AM, getting a lot of "removing from database" messages ....

yes I do have current sql backups, but I thought this thing was actually wiping nearly all images from the harddrive, until I checked the data folder.