Following yesterday's maintenance, we have resolved this issue. Basic Auth is also available again for the Dynamics Connector. You may need to re-connect to Dynamics if you haven't already done so. To connect with OAuth (Non MFA) you can follow these instructions.
If your organization requires MFA you will want to connect using Basic Auth.
If you continue to experience issues connecting Act-On to Dynamics, please let our support team know here
so we can investigate further.