Quarantine Digest stucks

Hello,

on our Sophos XGS 126 with SFOS 20.0.2 MR-2-Build378 we have the problem that all quarantine digests get stuck in the mail queue.

All other mails are delivered without any problems.

In SMTP logs I can find:

R=router_for_notifications defer (-1) DT=0.000s: lookup failed for all hosts in router_for_notifications router: host_find_failed=ignore host_all_ignored=defer

routing defer (-52) DT=0.000s: retry time not reached

I do not see any incoming quarantine digest mails on the exchange server.
But normal mails are routed there without any problems.

Does anyone have an idea where the problem could lie?

Many thanks
Dennis