I had a similar problem last year at a festival. I was using the USB stick to record (something I rarely do). The board was not on a UPS. I had owned the board for 18 months, and used it without a single problem up until this crash.
When the board crashed I received an error message. Like yours it said I had to reinitialize all memories. I clicked OK, but the board just ran through it's strart-up to the part about "checking USB stick", and then rebooted. After this happened 3 times I pulled the USB stick and the board started up, initialized to factory settings. I spent the next 10 minutes getting the board re-configured.
I know exactly how you feel about losing trust in the mixer. I was about to get rid of it in the days following the festival. The good news is: After over a year the problem hasn't returned.