MailChimp came back and said it appeared to be a problem on their end:

Laura (MailChimp)
Feb 10, 1:17 PM EST

Hi Mark,

Thanks for following up with this. This time I was able to get some information from the API , and it looks like the issue is actually something on our end. To be totally transparent, I am not sure exactly where the issue lies, but it appears to be related to unsubscribers of a particular campaign. I went ahead and passed these details to our developers so they can do some investigating. While I'm not able to provide a timeframe for the fix, you're welcome to check in any time with ticket ID 1668389 about our progress.

Again, thank you so much for your patience while we work on this.
Thank You,
Laura

Now, when I log into Powerbi.com, I get this:

https://portal.office.com/BoxError.aspx?aspxerrorpath=/default.aspx

Server Error in '/' Application.

Runtime Error

Description: An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page for the first exception. The request has been terminated.