Unable to Import Linux Reporting Server into CM (10.40)

New Data Protector CM (10.40, Windows 2016), Linux IS (Centos7), supposed to act as reporting server, too

No problems with installation, but import fails after pressing "Validate" with "Data could not be fetched".

Iptables are cleared. We checked communication between CM and RS with tcpdump, and there is some traffic, so no basic network problem. Test with  RS on Windows server was without problems.

Incidently, same error occurs when invalid credentials are used. 

Any ideas, anybody?