Not booting with second Screen connected

I used KDE ages ago and started a new Linux journey lately.

As I liked KDE a lot I use cachyOS with KDE Plasma on my Razer Blade 14 Laptop.
The thing is that the system won’t boot with a second screen connected.

I use a DELL U4021QW and its internal Hub for Keyboard, Network and Loading with the Thunderbold connection. All of these are working fine when connected.

But as soon as I do a restart or just start the system while connected it won’t boot.
I see the Bootloader Screen and afterwards only the turning circle on and on.
When I disconnect and boot again it works and afterwards I can connect via Thunderbold again.

Is this kind of a known issue or is there something set up incorrectly?