I'm having the same issues and I just went through the steps you provided. I'm only seeing one active session (My current Macbook Pro session) and I've only installed GM2 on two computers ever. I've tried multiple times to revoke the session and retry, but I get the same error every time. It looks like I have the right version installed as well and I've tried reinstalling twice. Is there a way to install a different version?
When I do install GM2 I'm seeing a lot of extra files in the application folder that typically don't show up when you install a new app (Files named "buildNumber", "snippet.xml", "update-mac-I.rss", and a few others) and I'm wondering if that's somehow related? I'm currently running El Capitan and I can't try Sierra since it's not supported on this hardware.
I've also opened up a ticket for this yesterday, but I haven't received any response yet.
Edit: I just tried to install older versions of GM2 by just changing the numbers in the link that was provided above that matched numbers based on the release notes. It seems to login successfully, but tells me to download the latest version and doesn't let me do anything else. One thing I did notice is the 2.0.7.173 version is the only version that creates those weird files in the application directory that I mentioned so maybe this error is related to that.