Unable to deploy Mobile Config for Active Sync on iOS 13.4.1
As I have identified, the issue was that the current iOS versions (13.4.1, iOS 13.4.5BETA) have a bug, where iOS can not use client certificate authentication.

It seems like this was finally fixed in the iOS BETA 13.5 (17F5065a).

With the client certificate authentication working again, the setup worked on the newer iOS version.