It could be some bad welds that have come off the Cells which are causing the BMS to misbehave.
The only sure way is to open the pack and measure each cell row directly and via the balance wires for any inconsistencies.
Also, inspect the welds.
If you don't want to risk it or whatever I wouldn't mind taking a look at it for you if you are willing. I also have a few 20S BMS.