Problem found and solved myself.
Since the "Agent Desktop Servers" address field was empty, I looked at the Network (name/IP addresses related) part of the server.
This CallManager was already running in production, with Network teaming (NFT mode) enabled (!), which I didn't know before the IPCC Express Installation.
Turning off the Network Teaming and re-install the IPCC Installation again, solved the problem mentioned.
Conclusion: Check if Network Teaming is disabled before installing IPCC Express!