I'm on macOS 10.13.6 and Mail just started to prompt me to authenticate to connet to my two gmail accounts. So I did, using two-factor-authentication, everything seemed to work fine until the last step, when the window contents cleared, but the window would not close. It just sits there, has a cancel button and nothing else.
I can wait forever, the window will not close, I can force quite Mail in which case I need to restart the process with the same result or I can click Cancel ("Abbrechen") in which case I also would need to restart the process.
I even tried to remove the account from the OS entirely and then to re-add it, but it also lets me not finish this process. It sits at this window then and does nothing:
I clicked "Finish" (Fertig) there, then the button grays out, but the window just stays there and the account is not added eventually.
How can I resolve this issue and connect to my Google accounts again. Updating my OS is not an option.

