Answer
From a certain Apps10 update onward, Outlook Calendar synchronization (delivered through the RMPANL add-on module) requires a one-time manual setup step beyond just enabling the feature: installing the UpdateSyncMasterUser.sql script and unlocking the dedicated SYNC_MASTER database user. Without that setup step completed, calendar sync attempts fail with this SYNC_MASTER-related error; once the script is installed and the user unlocked as documented for the Outlook Calendar Synchronizer add-on, the error clears.