Target Display Mode: Flickering Screen

Hello there

Just bought a new MacbookPro 2018 15" and wanted to connect it via the Target Display Mode with my Mid 2011 iMac 21.5".

Of course I had to get an adapter... I did so and connected the Thunderbolt 3 (USB C) to Thunderbolt adapter with a Thunderbolt cable. Connected both the Macbook and iMac with it. Both running High Sierra 10.13.6.

After pressing Cmd + F2 there is some strange screen flickering going on. I actually can move the window from the Macbook to the iMac and "work" there. But the whole screen of the iMac is flickering like crazy and going on-off ever half a second. If I try again, it sometimes doesn't even show anything...

Do you have any help?

I am so frustrated! Already did reset the PRAM/NVRAM on the iMac. Should I do the same on the new Macbook?

Thank you so much!

Lukas