Hrm,... no clue how I managed to break my Gallery installation. I probably played around too much with gallery2flickr. Though maybe I killed it when bumping and testing the ebuild last time.
Anyhow it must have been a while ago so I actually had to ask dar to go into recovery mode. And since this happens very infrequently I always forgot how to select a sub-path of the whole archive dar creates.
Here is my reminder:
dar --crypto-block 20480 --key MYKEY -x 01-18-2008_0358 -g localhost/htdocs/gallery
Hello again, my dear Gallery... Backups are indeed a nice thing.