Can't shutdown on Dell Optiplex

I'm currently running into a problem on multiple Dell Optiplex devices.
I installed Flyby11 classic on 3 Optiplex's and i can't shutdown any of them. Doesn't matter if i do it over the windows menu, the command prompt or the power button. In the end i always have to pull the plug because it keeps on running.
When shutting down it shows the normal "windows shuttding down screen". Then the screen turns black but the system keeps on running. The LED on the front of the PC indicates no traffic on the hard drives tho but i hear all the fans running.

Any fix for that? I already tried turning off 'safe start' (idk if its called like that in english). :)

The error occures no matter if i do a fresh install of windows or if i upgrade from 10 to 11.

Btw. the error didn't happen on any other PC or laptop from Dell. Just the Optiplex series!

Thanks for help!