Seems I made a very bad mistake during ma research for #2327 “Wiki Plugin for Kanboard: Attachment handling”. I seems that I lost all projects and additional data in one of my KANBOARDS, probably by some database violation.
I have a backup of the canboard installation, but probably that will be useless without a database backup?
With Database Backup from Provider (1/2 h before my mishap) and a little help by Alfred I was able to restore data. Resolved!