Update as of 04:55AM UTC 13 November 2018:
Service Disruption has been resolved and full service has been restored. Please contact Support at http://success.pardot.com if you encounter any further issues. We sincerely regret any inconvenience you have experienced as a result of this incident.

The Salesforce Technology team is currently investigating a service disruption impacting a subset of customers as of 03:05AM UTC 13 November 2018.  Updates will be posted here as new information becomes available. We appreciate your patience while we continue to investigate the issue.