MSSQL Mirror Configuration Troubleshooting: Witness server forcibly disconnecting every 100 seconds
We reported the issue to Microsoft support and got it fixed. I have also tested SP1 CU1 and the database mirroring error messages have disappeared. So I think that all is good.
https://support.microsoft.com/en-us/help/3210699/fix-error-messages-are-logged-if-database-mirroring-is-configured-by-transact-sql-in-sql-server-2016-and-no-database-activity-occurs-for-more-than-90-seconds