ActiveSync authentication issue with a single mailbox

Exchange 2013 Server Mailbox
 > Apple iPhone 6
 > Samsung Galaxy S3

Hey folks, I have one user that is unable to connect via the mobile devices above to her Exchange 2013 mailbox using the native clients on the devices. we have about 12000 devices, but this is the only mailbox experiencing issues.

Does anyone have any ideas where to start troubleshooting?

I have tried logging in with OWA (Outlook on the web) to verify the credentials are correct, they are, OWA sign in works, Outlook Client works.

I have tried logging into different devices, all get the "authentication failed" or when using the Outlook App "Unable to log in, Please check your email address and password and try again" message.

I've compared the AD account and mailbox next to another users and the settings look identical.

Is there a way to reset the EAS settings on a single mailbox?

Thanks in advance.
Christian HansAsked:
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FedCommented:
1. Make sure to remove all devices from "Mobile device" on ECP -> manage other mailbox -> mobile
There is a limit of how many Active Sync devices you can have attached to a mailbox

If that didn't work do the following.
2. On active directory, look for that user and open properties. Check under security for permission inherence ON
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