Mailbox monitoring does not work anymore, after changing the language of the Google account to another language.
If you have created a configuration for Connect To Mail with a Google account, with mail language „english“ and you change it to another language e.g. „german“, it could happen, that Connect To Mail does not find a folder to be monitored. This is because the folder name like „Inbox“ changed to „Posteingang“.
Change Language in Google Global: https://support.google.com/accounts/answer/32047
Change Language in Google Mail: https://support.google.com/mail/answer/17091
To solve this issue, create a new Connect to Mail configuration, or reset the language for Google account/mail-language to the previous language.