Kemper shutoff and wouldn't come back on for an hour

I use an inexpensive APC battery backup unit with mine - all the time. If the power goes off for more than just a blip, you can shut down your KPA normally. If it's just a quick outage, you're up the entire time.

Only the mothership can answer that one! Meanwhile, I'd get the battery backup ...

A few questions, Paul, based on the possibility that your rehearsal space gets quite-warm but never, until now, hot enough to prevent power-up:

Did it get warmer in the rehearsal room last night? Did you place things next-to / around / on top of the unit that you don't normally? A sheet of paper over grille slots could conceivably make a difference if you're already in the ballpark of the threshold.

As I said, it's just a possibility I wanted to alert you to, unlikely as it is given the KPA's tolerance for heat and minimal thermal footprint.

There's an automatic breaker inside the KPA to protect against power surges. You must have tripped it. In order to minimise "downtime", support recommend pulling out the power cord from the back of the KPA, waiting 20 minutes, then plugging it back in. I've only read a few posts over the years about this happening to people, mostly in the US. Seems you have more power problems over there than here in Europe.

For self resetting breakers, a PTC resistor is often used.

These need some time to cool down and "reset". No idea if this is the case in the Profiler but it would explain the time it took (although 1.5 hours seems to be a bit too much).