Beta 7398 Administrative logs filled with errors ...

I have thousands and thousands of these in my administrative logs ... I can't even tell what they're correlated to ... The entries just before these items range from immediately after the previous log entry (within a couple seconds) to up to a minute after the previous log entry.

23:16:50.970 TlRMTVNTUAABAAAAA4KIAA== Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: startIndex
   at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
   at System.BitConverter.ToInt16(Byte[] value, Int32 startIndex)
   at MailService.Protocols.Authentication.NegotiateMessage.TryParse(Byte[] data, String& failure)
   at MailService.Protocols.Authentication.NegotiateMessages.Parse(Byte[] data, String& response, String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache)
   at MailService.Protocols.Authentication.AuthenticationManager.Parse(String base64, String ip, String hostname, ProtocolTypes types, Boolean removeFromCache)