Google authentication failing due to unseen CAPTCHA

It appears the root cause it not the upgrade, but a Captcha I am being presented with but unable to access.

To narrow down the issue (which was authentication for all my calendars) I turned off all my "Online Accounts" calendars and kept only one Google calendar that I had created manually.

I then disabled the calendar to keep it from prompting me.

Next, I went into the Properties and tried to retrieve the calendar list to stimulate a re-prompt for password.

What followed was a dialogue I have not seen before, which stated "A CAPTCHA must be filled out to log in".

I am not sure how to resolve this, as I don't require it when browsing in chrome as my session remains valid in my browser when looking at my calendar using the same username.

Screenshot of Captcha prompt in evolution

If you think your authentication failure may be due to CAPTCHA, try using the unlock URL to tell Google you are human.

https://www.google.com/accounts/UnlockCaptcha

I have not had a re-occurrence yet, but online docs suggest it will work.

The only way to confirm auth failures are indeed CAPTCHA (and not some other issue) is to disable the calendar (to stop the password prompts) and then navigate to the Properties and Retrieve the Calendars. It seems that this invokes the libgdata interface that will report back the CAPTCHA failure.

According to the upstream developers the CalDAV interface being used does not properly support the CAPTCHA exception, so this issue will remain until things cutover entirely to the new auth mechs in libgdata or Google updates how the CalDAV interface works.

Similar issues happen to iPhone users who integrate Calendars via CalDAV:
http://www.google.com/support/forum/p/Calendar/thread?tid=78dc7e4076e4fe26&hl=en

Another workaround if the unlockURL does not work may be to hit the CalDAV interface in your browser and see if it gets unlocked there (and capture any undocumented CalDAV failure exception).

Server: https://<email address hidden>/user/

Note: if you are trying to sync a secondary calendar, use this URL

Server: https://www.google.com/calendar/dav/[string of numbers and letters]@group.calendar.google.com/user/

I have confirmed that to solve this problem of the password prompt re-occurring endlessly, just go to the following URL and complete the CAPTCHA.

https://www.google.com/accounts/UnlockCaptcha

I am not aware of any other solution other than to wait it out (goes away after some time) and/or switch networks.

I have had this issue occur on both Rogers and Bell networks which likely rules out an ISP sniffing issue as the cause.