I noticed that my Gmail Notifier always return the follow error in these few days:
An error has occurred.
Cannot connect to your mailbox.
Service temporarily unavailable
At first I suspected the relevant server was down but it's really strange that it lasted that long. There must be numerous complaints if it is really the case so I did a Google search using the above error.
Then I found the root cause. It is because I have enabled the "Always use https" setting in Gmail a few days ago. Oh my god, I do think this is a really good feature but I didn't expect it affect the handy notifier... Before having this setting I had to modify the URL to start with "https://" manually each time I was redirected out of the login page, which was quite inconvenient. So no way out, I have to gave up the notifier to enhance the security of my account.
Oh yes, I know that Google chat provide that feature too, but I just don't want to keep it running all the time.
Gmail Help Discussion: Gmail Notifier not working