Sound like an out of range imbalance for at least one of the four cell modules. Particularly if the fix is a new pack.
A good BMS (so I assume Alta's as well) should actively balance each of the 21 parallel sets of 6 cells (126 cells) in each of the 4 modules. This occurs during charging and discharging. That means balancing a total of 84 parallel sets of 6 18650 Li-Ion batteries (cells). If, either as a set or at the module level, the voltages get out of spec it probably throws that code. Discharge, particularly during hard riding, will put the biggest strain on the balancing system as it is constantly shuffling current from the higher voltage sets to the lower ones to "pump them up".
The inability to maintain balance (out of normal range) can be due to a bad battery(s) within a particular set, an open/intermittent circuit (there's literally over 2000 wire bonds alone), or a problem in the balancing system itself.
Mfgs try to avoid bad batteries by properly testing and matching each before integration into the module but maybe some slipped by. Depending on the failure identifying the exact one(s) could be a pain without the proper diagnosis software. The PCBs can be inspected/tested for bad connections pretty easily. I suspect any "battery to board" issues could be fixed pretty easy. The biggest problem would be an issue with the components on the modules circuit board, although most are commercially available the flash data inside the individual micro controllers is not.
Just based on the percentages/numbers, I'd say a bad connection or two. That can be fixed in your garage.