Forum Discussion
Exchange Office 365 - Delegate Calendar Issues
The good news is you have already done enough work to isolate down the problem to your mail client, and you have a date it started.
The bad news is there is likely not much help people are going to be able to provide you here on the Microsoft forums. Because you know when the problem started and that it is isolated to your iCal's I would try either the apple forums, or even opening up a support case with apple directly.
Having worked in IT support around email though, I can tell you that you have done 90% of the work with your testing, you just need to find vendor specific help now with Apple.
Here is the iCal support section directly for you - https://www.apple.com/support/ical/index.html
Goodluck!
Adam
Thank you so much for coming back to me.
I've gone down the Apple route and i've been told that nothing has changed with iCal or OSX to cause this.
I then tested my MacBook, and a secondary iMac and they are the same.
Everything is pointing to a permission change in Exchange that is revoking these third party requests.
So we have 3 iMacs, 4 Mac Minis & 3 Macbooks that no longer Sync. All on different OSX versions.