iPhone connection failure to exchange server

After the last server restart we have had several issues with our iPhones connecting to the exchange server. 4 users have phones which connect, two work fine and two have connection failure issues, but there are some differences in them.

User A is able to add the exchange account, it verifies but then gives connection failure. When using https://www.testexchangeconnectivity.com/ it gives the following.

Attempting to test the GetItemEstimate command for the Inbox folder.

ExRCA received an error in response to the GetItemEstimate command.

Additional Details An HTTP 500 response was returned from IIS7.

I have unchecked/checked the inherit permissions box, removed the partnership and re-added the account. Checked to make sure ActiveSync is enabled, and looked through every setting I could find that was listed as a common solution.

User B is also able to add the account and have it verify, with the same connection failure message. But with this account, it passes completely on the connectivity test and when I attempt to add the account on another iPhone it receives email just fine. The iPhone that does work is running IOS 6.1.3, and I have attemped resetting network settings and removing and adding the exchange account multiple times.

Both of these issues started at the same time, roughly 1 week ago and I have been unable to diagnose a solution. They have such a similar issue, but the first is likely something on the server, and the second something on that specific phone.