I've had a Mailchimp report that has worked for many weeks that has now stopped showing any values in the "Google Analytics tracking" column.
It's also intermittently returning "Error: Invalid query specification", though corrected itself today after retrying manually.
If it helps, the column is "yvEc9iPibj1styhZ6nLJ5E4GfgHU5R".
It seems that this problem was related to some performance improvements we made inside the past couple of weeks. Apologies for that. We deployed changes that restored previous functionality and it seems to have done the trick.