Hi,
installed are the following components:
- vSphere 7.0 Build 15843807
- vCenter Build 16189207
- Connection Server on Server 2019 with Horizon 7.12
- Windows 10 (1909) with Horizon Agent 7.12.0
Attached are the logfiles from the Linked Clone VM, thanks for your help!
I never had such problems with a Horizon 7.10 installation on vSphere 6.7 with Windows 10 1909 
Kind regards
Sebastian