How do I restore my app (K-9 Mail) settings from an adb backup?
OK, some googleing later I found Titanum Backup and that it supports restoring selectively from adb backups. I copied the backup to my SD-Card and then restored using Titanum Backup and and just worked. (However I'd still be interested in what I missed in the first place).

Note however that the free version does not support restore from adb files.