I installed firmware-misc-nonfree. But it didn't help. I tried different suggestions I found. More editing on Grub f. e.. But is also didn't help.
As far as I understood it's probably a kernel issue.
Patching firmware myself is a step to far for me. I'm still a linux beginner.
I will wait for a new kernel and hope, that the issue will be gone then.
Thanks for your help.

Unfortunately the issue reappeared. I could not reproduce it for a while, but since today it's back. And it makes no difference if an external disk is plugged in or not.

I noticed the issue directly after upgrading from amber to byzantium. I did not update coreboot at that moment. I'm not sure about EC firmware. I think I can not install/update this on my Librem 15v3.