If you try to add a google calendar and see an error message like this before you even get a chance to login:
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Then it's likely that a popup blocker, or other content blocking mechanism has prevented Marvin from connecting to your google account.

If you're using Firefox, the built-in tracker blocker may be causing this issue:

First click the button at the top-left of your screen next to the URL input, then click "Turn off Blocking for This Site" in the popup.
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