I’m running Urbackup 2.2.4 beta on a Debian 9-System but this also happened with the latest stable release.
Regularly the urbackupsrv-Service gets stopped. In the logfile there’s this entry:
2017-10-14 07:10:37: ERROR: Timeout during flush request
The server won’t make backups and the webinterface is not accessible until the service gets restarted.