Managed Availability ActiveSync\ActiveSyncCTPProb: WARNING: Index was out of range.

I'm running into a Managed Availability issue. My server has the ActiveSync health set Unhealthy. After investigation I noticed the following error when the probe runs:

WARNING: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

Command used: Invoke-MonitoringProbe ActiveSync\ActiveSyncCTPProbe -Server <servername>

I was not able to troubleshoot the issue any further from this point. This happens on all servers, Exchange 2013 CU7. I did not make configuration changes against the EAS virtual directory. Reboots, restarting services and waiting did not solve the issue. The information on https://technet.microsoft.com/en-us/library/ms.exch.scom.activesync%28v=exchg.150%29.aspx did not help either, because I'm stuck with the error message.