I am now using a 5V/1.5A power supply.
So...it's been 48h till now and still I didn't get any new reboots. It seems, that this problem is solved. Also 2 new scheduled backups were made.
Thanks for the tip.
How would you evaluate this. At the current time I am potentially using more volts. 
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Undervolting was an old way to slow a processor down. So I'm guessing overvolting is a good thing!
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Still used on Raspberry Pi.
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5 days without a single restart.
Someone should pin that topic or add this solution to the docs/FAQ.
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Great solve. I have the same and after reading this I immediatly changed the power-supply, its about the last thing I would think about frankly. I was already looking into apps/rules/etc.
Thanks for the solution. (now lets hope it works). 
