battery was disconnected and not charged from any reason (fuse exchange etc..)
battery voltage is too low
wiring error
battery cells highly disballanced
BMS issue
Quick solution
plug BB3 to AC wall plug or solar charger for a few minutes and check voltages. If supply voltage is within range, disconnect charging and check if it starts normally. If voltage is too low, let BB3 charge for a few tens of minutes. Then, check again.
Detailed solution
If above proceedure does not help, proceed with battery cell check. If no issue with cells found, ask unitronic for help.