10.50 -> 11.01 on WIndows Upgrade Error: [CRITICAL] The digital signature of the object was not verified.

Hello,

I am trying to test the upgrade from DP 10.50 to 11.01 on Windows Server 2016

The upgrade runs through but leaves the Cell_Server in a desolate non-functional state.
The first line in the installation summary shows a critical error:


[CRITICAL] The digital signature of the object was not verified.

Does anyone have an idea where this error comes from?