A severely degraded battery can cause voltage dips that glitch internal logic.
Some APC Smart-UPS with network management cards (NMC) can experience firmware corruption causing spurious Error 02000. Connect via serial cable and use APC PowerChute software to reflash firmware. Note: This only works if the UPS powers up for at least 30 seconds.
Do not open the UPS. High-voltage capacitors can hold a deadly charge for days.