I am running latest version of C5 (8.5.5) and lately I cannot connect to community to Extend C5. When I try, it gives me a very small frame window with another login window. If I try and submit this form it gives Invalid Token error. However, the project gets created under my C5 account under concrete5.org, but cannot assign licensees to it as my local installs don't see it as connected. I have this problem on two separate websites. I have tried 3 different browsers, tried incognito mode and cleared cache and nothing works. I have seen others having similar issues but none of those fixes worked. Looking for assistance.
One thing you might try (unless you have a Mac with Safari sitting around), is to open up the inspector window in Chrome (F12, or right click and Inspect Element). Once this window is open click and hold over the refresh window icon and you get three choices. Choose "Clear Cache and Hard Reload" on the page. I have found recently that browsers (especially Chrome) hold on to stuff for a while. Hopefully this may resolve. Other option, if you have iPhone, then go to the page on your phone in Safari and do it. Another side note, even though my own website wasn't being recognized as joined to the community, each time I tried it was creating an instance of the website in projects part on concrete5.org in my account. I ended up with multiple duplicate projects in my account. Good luck.