When I login into Microsoft teams, it should redirect me to my account.
After the first screen where I insert my mail account, it appears to redirect me to my organisation URL (which is pretty normal), and when this happens, the screen turns grey (See screeenshot).

I'm just trying to sign in
I'll close this issue as now the service is working properly.
Same problem here I'm using Franz 5...
At the end my problem was that my network had a proxy, I don't know if this could help you
I tried access in my home, the trouble continues.. :(
Version 5.0.0-beta.18 (5.0.0-beta.18.903)
os: macOS 10.12.6
The same issue. As soon as you reload the service it's asking to put your username again.
Same problem here, when the login screen tries to redirect you to the login page at login.microsoftonline.com the screen is waiting infinitely. Not turning grey as the original post, but the same effect .. no login possible
Franz version is the latest 5.0 Beta 19
OS: macOS 10.14.1
So Guys, I can tell you what worked for me.
I had this problem when I was behind a corporate proxy, and apparently, the only solution is to configure that proxy correctly. After that, you should be able to see the window working.
We don't have a proxy.
Well, I checked it today one more time and now it works.
I haven't changed anything but who knows what was done behind the corp sense. Maybe iT opened something maybe something else.
Most helpful comment
Version 5.0.0-beta.18 (5.0.0-beta.18.903)
os: macOS 10.12.6
The same issue. As soon as you reload the service it's asking to put your username again.