Jonathan, we are seeing the same issue.  I believe it probably has to do with an O365 outage that occurred this morning.  I submitted a support ticket and have reached out to a few POCs to try and expedite.  It seems that the O365 outage is resolved, but now something probably needs to be restarted or reconnected on the Nintex side maybe.

This is super bad because I am demoing the Nintex Mobile app to our CEO at 1:00 this afternoon!!!

Thanks Brent - sorry for your situation but glad it isn't me.  Did you remove and re-add to remove the Server Error in '/' Application Error and then get this issue?  I've tried recovering the license through the store and receive a message indicating that it is not available and will be restored soon - I agree it seems to be a nintex issue that needs to be resolved asap - I've opened a critical ticket with them but unfortunately don't have a support agreement so I'm just stuck waiting.

Reply from Nintex Support:

________________

Good morning.  We are aware that there are currently issues with O365 and we are investigating the cause.  As soon as we have an update, we will let you know.

Our apologies for the inconvenience.

Best Regards,

Appears to be working for me.  Thanks for getting us back up.