Thx... Checked them already. Meanwhile I succeeded with overclocking. That works now well as expected. But still there is this problem with extremely slow and wrong hardware monitoring in the bios. I guess something's wrong on the board itself. 84K rounds for the chassis fan - it's a astonishing that my rig isn't starting up and away like a rocket.
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Edit:
Just pulled the plug of the chassis fan and monitoring works as it should. So I assume the sensor is broken and causing the lags.
I also tried the other chassis + power fan connectors. They all seem to work not as they should. That's really annoying. But I don't see a way to use fan connectors on the board except the CPU fan.
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2nd edit:
Ha... I wasn't convinced of my guess, so I just tried another case fan. And tadaaa... Everything's ok. The fan is the thing that's broken - What a relief.
Ryzen 5 2600X - ASUS ROG STRIX X470-F Gaming - 32GB RAM - Nvidia GeForce GTX 1080 Ti