Target Display Mode not working

It feels like I have read all the threads on this, but have not come to a solution.

Problem:

All I can get the iMac to do when pressing Cmd+F2 is either to make an error sound or to flash (in Safe Mode).

Setup:

. MacBook Pro 13" 2020, macOS 10.15.7 (Catalina) as source

. iMac 27" late 2012, macOS 10.13.6 (High Sierra) as target

. Thunderbolt 3 (Usb‑C) to Thunderbolt 2 adaptor

. Apple Thunderbolt cabel

I have:

. started both machines in Safe Mode

. tried resetting the PRAM/NVRAM on both computers

. reset SMC on iMac

. tried connecting in iMac lock screen

. reconnected cables (to different ports)

I am lost. Any suggestions are welcome.

Best, Nils