Hi Volker,
can you take a trace on the sntp-Server ? It would be interesting if the B5 sntp request arrived the server or not. If the request arrived the server, you could trace on the B5-site if the server response arrived at the B5.
Take a reboot to restart all services on the B5 and check it again.
Otherwise -if the configuration and SW ist the same like all other B5, i would reset the B5 to factory default, reboot und configure new.
regards,
Carsten