If you accept the certificate, does everything continue as before? Rather than trying to figure out what caused this issue, I would suggest that you Up2Date to 9.304 unless you're in a High-Availability mode.
If neither of those works please post back here and also have your reseller open a ticket with Sophos Support.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005