[GMail] Connection timed out / Operation aborted

I've set up xfce4-mailwatch-plugin for a GMail account via Add -> Remote IMAP Mailbox. The parameters are:

Mailbox Name: GMail
IMAP Server
  Mail server: imap.gmail.com
  Username: my_username@gmail.com
  Password: my_password

...where my_password is a Google app password specifically generated for Mail Watcher, because my account has two-factor authentication enabled.

It worked for a week or so, however, just now I've noticed a red dot in the panel, indicating that something went wrong. When I checked the logs, I could see this:

[GMail] Connection timed out
[GMail] Operation aborted

What could be causing this?