GAPPS usually works just fine, but sometimes if you check your email too often you would get an error about a web login requiered. I found mysleft able to login from the web but still not able to use Thunderbird to check via IMAP. So to fix it, I try this:
https://www.google.com/a/yourdomain.com/UnlockCaptcha
Update: Looks like after migrate those services to the new platform this is the right url:
https://www.google.com/accounts/DisplayUnlockCaptcha