Similar problem, I scan the QR code, and my phone enters only to say "Countered, the max number of players has already been reached"

Bunsens fix worked. Friend of mine using his Macbook and an Ipad had to change the second ? to an & to be able to use the ipad as his camera.

I found an issue with the QR code, malformed URL if a not-logged-in user joins with their phone.

https://spelltable.wizards.com/game/7PAvHSO9tBGxrceoUPwjZ?camera=true?login=true

Notice the second ? in the URL, that should be an &. If you experience this issue, you can change that manually.

This is the easiest fix in the world, literally 1 character (maybe a few lines depending). Add me to the github and I'll do it for free, AdamKruschwitz on github.

Copied from the support ticket that I opened:
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While I know this is an experimental feature, I thought it was relevant to raise this feedback.

1. I logged in Spelltable on my Mac laptop and checked the option to use my Mobile Device as webcam.
2. Using my Android device, I scanned the QR code and opened a browser to use Spelltable
3. As a result, I was logged out of Spelltable in Mac and my mobile device was showing the normal game screen, compressed in the small screen of my phone.

It seems that instead of using my mobile camera, it was actually signing in with the mobile device to play on it. I with different combinations of browsers on Mac and on Android.

Environment:
Macbook Air (M1, 2020) with macOS Big Sur 11.6
Samsung S20 with Android 12 (One UI 4.0)
Browsers on Mac:
-- Firefox 99.0.1 (64-bit)
-- Google Chrome 100.0.4896.127 (Official Build) (arm64)
Browsers on Android:
-- Google Chrome for Android 100.0.4896.127
-- Firefox for Android 99.2.0