Since you say the unit was not overloaded, please post exactly what was
plugged into the backup portion and the wattage of each unit.

The failure probably was just a normal electronics failure. So far you
have had pretty good luck in terms of hours and use of the APC backup
units. You do realize that the one you had in the picture was the
cheapest bottom of the line UPS you could have been using?

Where I work paid $3200 for a real UPS power converter for a situation
where power interuption could be life endangering. It was a 3KVA ONEAC
with full power conversion.