My 2 yr old group has a single subdomain, which was added last year. I don't know if the start of this issue coincides with that or not.
When trying to navigate directly to a message thread (like clicking on the View/Reply Online footer link in a message email notification), if I'm not already logged in, it works fine. But if I'm not already logged in, I'm prompted to do so. Upon logging in via Google, Groups redirects back to the message, but drops the subdomain from the URL, resulting in the "Sorry, but that group does not exist" error.
This issue doesn't seem to affect either username/password or Email me a link login options.
Actual result: Browser redirects to the original URL, minus the subdomain (https://groups.io/g/main/message/1234), which isn't valid and Groups displays an error.