The Input selected on the Performer end must also be a mono port, just as well as for output. So if the menu offers stereo 1+2, blabla 1, blabla 2, you need to select the 2nd entry if the Performer has connected her microphone to blabla 1.
Also check https://helpcenter.steinberg.de/hc/de/articles/360012899079-VST-Connect-In-depth-information-and-troubleshooting