Exchange Password Incorrect Popping Up At Midnight

Environment: Windows Server 2008 R2 / Exchange Server 2010 SP2

Problem: There are 7 people in the organisation that currently have iPads, all fully connected to Exchange, with mail, contacts, calendars etc syncing perfectly fine. These people also generally also have their iPhones connected, they are all working flawlessly. There are 2 users though who’s Exchange accounts authenticate fine to the server and work, sync etc, but every night at around midnight a password prompt appears asking for their Exchange password, even if they enter it correctly it keeps popping up until you turn the device off and then on again and it all works fine. I’ve tried changing their passwords, factory defaulting iPads/iPhones, patching the Exchange Server to SP2…

It is affecting no one else in the organisation except for these two users.

Does anyone have any ideas?