Cannot install Software on W10 Client using user-account with domain-admin group

My Setup:

  1. UCS Active Directory compatible as 1. DC
  2. UCS Member-Server with Kopano (no AD capabilities installed)

I have created a few users for testing and added 2 of them to the following groups:
Administrators (Builtin)
Domain Admins
Domain Users
Enterprise Admins
Group Policy Creator Owners
Schema Admins

But I still can’t install Software on W10 Client using any of these 2 user accounts.

additional info:
lusrmgr.msc shows that users from MYDOMAIN\Domain Admins are in the local Administartors-group.

A reboot of the affected Windows-client fixed this issue.