Check all your USB ports very carefully with a magnifier and bright light.
Maybe there is dirt in there, or a bent contact?
I did a general visual check only. I will then check the ports more carefully and may even use my new can of compressed air
If you can, try and resolder the SATA connector
Unfortunately I can't solder, should do a crash training one of these days
turned out that the the ground plate of the (low quality) USB socket was bent and touched the voltage pin, or something like that.
When checking the mainboard yesterday it didn't look very dirty or optically damaged. But the TP came with two cracks in the palmrest, near the fingerprint sensor and one on the back, close to the vent holes. Maybe it fell down or took a hit that damaged some internal components. Luckily the screen is ok (lid is also ok, no signs of cracks or damages), modem and Wlan card seem ok as well.
Thank guys or the hints!