Exchange 2016 account will not perform activesync. Setup but no connectivity

The issue is that on some accounts that I create in Exchange 2016, they cannot get email on an iphone.

I'm using exchange version 15.1 (build 225.42) running on Windows Server 2012, on premises.

I have created a number of accounts via exchange's web EMC. They also exist of course in active directory.

Some (most?) accounts that I create have no problem with this.

However I've got one account that I created some time ago, it works in outlook, works in OWA, receives email just fine, but cannot get email on ANY iOS device. I have not tested on android.

On any iOS device that I set this account up on, I can set the account up, autodiscover works fine and appears that everything is good, but I can tell that it won't work because "Automatic Reply" in the account screen just says Loading.

I can set other exchange accounts up on this phone. If I set this account up though on other phones it does not work. Thus I believe I've eliminated it being a phone specific problem. Some test accounts that I've created (with the same methodology!) work. I'd like to figure out why this particular account won't. When I go into mail for this account on the phone, after a moment I get "Cannot get mail/The connection to the server failed." And the phone does not show up in EMC under Mailbox Features> Mobile Devices.

I've done get-mailbox with | fl for the account, and compared it line by line to other working accounts, and cannot see anything different.

So:

Has anyone run into this sort of thing? Is there a bug or misconfiguration of some sort that people have seen before?

Or does anyone have any pointers on trying to get in deeper on this? Are there activesync logs that I should take a look at?

Thanks!

edit: activesync is enabled, according to EMC and powershell.

MS testconnectivity site gives passes except for:
The test of the FolderSync command failed.
Exchange ActiveSync returned an HTTP 500 response (Internal Server Error).

It linked me to this article:
https://technet.microsoft.com/en-us/library/620c5ce8-3595-4658-9a7a-ec76c10e4a69.aspx which solved it!

Thanks!!