Cleanup after backup storage loss

Server: 2.1.19 on Ubuntu 16.04 (Kernel 4.8.0-46) guest on KVM raw image also on BTRFS (CoW disable for the disk)
Client: 2.1.15

My backup storage filesystem got corrupted and now I have an undefined state of UrBackup and on some clients.
How do I reset UrBackup without losing my client configuration, only the backups (I want to replace the filesystem).
There are still /.urbackups_snaps directories with content äh mounted(?) on my UrBackup server and on some of my clients. How do I get rid of them?

Run remove_unknown. Snapshots on clients should be cleaned up with the next backup.