Thanks for tracking this down to this extent. The cause may be a stray newline in the configuration coming from the client. If you are using a Linux server you can correct it like this for existing clients (requires restarting afterwards):
echo -e "UPDATE settings SET value='ALL' WHERE value='ALL\n' AND key='image_snapshot_groups';" | sqlite3 /var/urbackup/backup_server_settings.db
Will fix it on the server and client side with the next versions.