Hello SnikZ,
I tried to do that in safe mode and also using a test user account and I had the same behavior.
I also made a clean install of macOS Big Sur on my Mac to test if there was a problem on the macOS and the behavior persisted.
I recovered all the data from a recovery tool and after I formatted the external drive it was working.
Before the problem on the external drive, I put the Mac to restart and when it started the restart the Mac presented me the following screen:

I had to turn off the Mac by pressing the power button for few seconds.
After it happened the Mac tried to boot but restarted again. And only after all this it turned on again.
Now I'm pretty sure that some problem on the macOS cause this but as I reinstalled I don't have the logs to get in a deeper analysis.
I just would like to know what's this message means to think what could cause the problem.