@jtk - To be clear, is this user a guest editor by your setting the board’s Share settings to Anyone with the link → Can edit? And they are they trying to join an existing video chat session? I ask this as guest editors are unable to start a video chat session as par the Collaboration with Anonymous Guest Editors help article:

Next I would ask if you have gone through the settings/troubleshooting with your client listed in the Video Chat help article? There was a fairly recent change where users are supposed to be able to select a camera and microphone right from within the board vs browser/OS settings, but I can’t find that post right now (was in in the forum, miro.com/changelog, What’s new popup in the app, etc.?)

https://help.miro.com/hc/en-us/articles/360017572214-Video-Chat

“is this user a guest editor by your setting the board’s Share settings to Anyone with the link → Can edit?”

Yes.

“are they trying to join an existing video chat session”

Yes

@Robert Johnson It worked fine with an iPad.

@jtk - I joined a video chat as a guest today myself through the Windows 64-bit desktop app and then through the latest Chrome browser. Did it work fine with an iPad for the same user? If so, on the same Wi-Fi network? Was the Mac wired or Wi-Fi? VPN? Firewall? So many things to check.