I wish the Zero app would reveal the distinction between the main powerpack and the Power Tank in any way, whatsoever.
The cel imbalance reading is unclear to me whether it is showing for the powerpack, the Power Tank, or the maximum value read from both.
And what it will not show is a voltage difference between the powerpack and the Power Tank, which is what causes the above sequence of messages.
Try to determine the voltages of each battery via the MBB console or the logs. If they're more than 3V (I think) different, the contactor of the lower one will not shut to protect from inrush.
For what it's worth, "precharge decay too steep" could be a real/separate concern. Precharge comes from the BMS onto the main bus to put enough voltage into the motor controller's capacitor banks to avoid significant current outrush. Decay happens once precharge ends, and is measured for safety against there being an issue in the controller or on the main bus. I don't know how to read more into that, but there's what I understand from a first pass.
I generally recommend a slow (1-3kW) charge to 100%. The batteries need a slow charge in order to avoid transients as the lower battery reaches the voltage of the other battery, to give it a safe window in which to close its contactor and get the batteries balanced.