Hi a_reck,
Please make sure the Enhanced Mode is allowed in both Enhanced Session Mode Policy and Enhanced Session Mode in Hyper-V Settings.
Open services.msc and restart the Hyper-V Virtual Machine Management service (vmms) and Hyper-V Host Computer Service (vmcomputer).
Try turning off "Windows Hello" in Settings -> Accounts -> Sign-in options.
Hyper-V Login in enhanced mode not possible - Microsoft Q&A
Also make sure the Integration Services are up to date.