iOS Exchange 2013 connectivity issues after ESEUTIL

Hi Everyone (First Time Questioner) We have been quite unlucky recently and had 2 scenarios which have caused us to need to run an ESEUTIL repair on our Microsoft Exchange 2013 environment due to dirty shutdowns. The Utility has successfully sorted out the issue on both occasions, however on both occasions an number of iOS devices seemingly random can no longer connect to the exchange environment using the inbuilt Mail App. This isn't on all devices or all users specifically but it seems to be when a certain device is linked to a certain mailbox, it can no longer access. E.G: User 1: accesses email on Work Computers Work iPhone and iPad everything is working fine on the iPad and Work Computers but the inbox keeps coming up with a server connection error on the iPhone. Users 2 - 10 no issues at all (all have iPhone and iPad and windows PC's) and then another random user has the same issue. there no correlation alphabetically, by mailbox size, usage or age. (typically it seems to be hitting the people who like to moan the most but I don't think that's a useful correlation. We have even run active sync tests using testconnectivity.microsoft.com and we get all green on there as well.

Now we are aware of the device number limitations within exchange and have cleared all old and existing devices and reconnected but still nothing. You can still send emails from the phone when the account is signed in, it will not however synchronise any of the folders to the device. We have installed outlook for iOS on the device and it works fine, but users are moaning about using this because its not what they are familiar with it (sigh). having spent hours on the phone with apple and still getting nowhere I'm stumped .

Does anyone know what may be causing this or how we can purge any mention of the device other than the remove deice active sync option within the ECP?