Mirror Panel signs you in with your work email and password, or with your Google account. Either way you end up in the same workspace, and your session is restored automatically the next time you open the app.
Signing in hands your browser off to Google and back again, and the return trip carries a one-time ticket that only the tab and the web address you started from can read. If that ticket cannot be found, Mirror Panel stops rather than guessing, and shows this screen.
The message names the two things that cause it, and the link takes you straight back to a fresh sign-in.
There are two common reasons. You started signing in in one tab and finished in another, or you began at a different web address than the one you came back to. Choosing Return to sign in starts a clean attempt and clears both.
Return to sign in lands here, where you can retry with either method.
Reloading the page the sign-in returns to, or stepping back into it, no longer reports a failure. If your session is already live, Mirror Panel simply carries you on to where you were going.
Start and finish a sign-in in the same tab. If you keep a bookmark, bookmark the address you normally work from and sign in there.