Feedback Loop? when Play or RECORD buttoon is pressed?

Having the problem that when starting to press PLAY or RECORD button at the control room side then the performer hears loud noise (similar to a fax machine sound or a feedback loop). He hears on top of this noise the normal song.
What could be wrong?

Cubase Pro 10.0.60 and VST Connect

I checked everything from the VST Connect troubleshootin guide (" In depth information and troubleshooting") without success.