SchalkB

B is for my surname, Botes. Not very original nick, I know. But created profile after a very late evening of faultfinding ... 😄.
 
 
 

That alarm comes directly from the BMS of the battery (communicated via CAN-bus). The Venus-GX is just relaying it. The "Battery monitor (512)" bit tells you the source of the alarm. The BMS on a Victron system almost always has deviceinstance=512.
In some cases I've seen internal alarms caused by a cell imbalance, but that was with a different manufacturer (interestingly, this other manufacturer uses the same CAN message for alarms and warnings, so maybe there is something there). The fact that a short disharge stint (failing the mains) resolves it seems to agree with this observation: Cell imbalance.
The pylontech protocol also doesn't have a specific flag for a cell imbalance (batteries that use the more standard 0x35A message do), so that is another reason to think it might be piggy-backing on this alarm.

Cell imbalances should disappear given enough time. The Pylontech balancers can only pass around 50mA... so it might take a really long time.