Caught in a loop: Startup Security Utility won't recognize administrator

Thanks for this hint (I had stumbled upon it before as well). I just ordered the proper Apple cables and adapters for the necessary connections.

But I would rather try to avoid that since setting up the iMac Pro has already taken up hours and days. I didn't mention that I already did have similar problems before - immediately after migrating my data and settings from my latest clone copy, the new iMac Pro acted up (it seems the problem was that the new iMac Pro had an older Mac OS version than the old iMac...). Then after days and actually bringing back the new machine to the dealer it seemed to work - until the "FileVault problem". Activating FileVault has been the last step in my long list of things to do to get this machine running (data verification, software license issues...). I'm working in a relatively complicated setup without "free" internet access which further complicates issues when trying to solve this problem. (I actually hauled the new iMac Pro to my home where I have a "free" internet connection).

Apple Support is on my phone daily now - but they didn't come up with a solution until now as well... I managed to get the log files from the iMac Pro and they are being transferred to a "technician" within Apple now - we'll see.