Symantec Endpoint Protection Manager (SEPM) upgrade fails when Database Mirroring is enabled.
2021-01-07 08:23:10.879 THREAD 96 SEVERE: com.microsoft.sqlserver.jdbc.SQLServerException: The operation cannot be performed on database "sem5" because it is involved in a database mirroring session or an availability group. Some operations are not allowed on a database that is participating in a database mirroring session or in an availability group.
This issue is fixed in Symantec Endpoint Protection (SEP) 14.3 RU1 (refresh) build 3385. For information on how to obtain the latest build of Symantec Endpoint Protection, see Download the latest version of Symantec software here.
Disable database mirroring, perform the 14.3 RU1 upgrade and then re-enable afterwards.