Please find more information here.
How can I start the meeting? I only see this:
Fine, problem solved!
It looks like you have started a meeting (the toolbar in the left-right corner of your screenshot).
Also it seems you have joined (in parallel) to another meeting (green window). And that another meeting is not started yet, so you are in "waiting room" (green window), waiting until meeting is started by someone.
I hope it helps.
Maybe that's the reason. I tested a lot. But how can I find out if there really is another meeting and how can I cancel it?
Click on "window close button".
This doesn't help. Just tried it. After then starting a new meeting I find myself in the waitingroom again.
In fact I can start a meeting with a participant but the green window remains. I see the videos only in mini size in the Blizz panel.
Go to options and click on "Use new meeting panel", Apply.