Why has my Galaxy S6 edge stopped synching with my exchange calendar?

[AC Question]

I have synced my phone with my work calendar (which is an exchange account) since I got the phone last July. It stopped working recently. I have removed the account and added it again. I have spent one hour on the phone with Samsung but still it is not there. When I go into Manage Calendars it is not listed but when I try to add it in in that screen it tells me it is a duplicate. I tried installing a third party calendar (a couple of them consecutively) but they only seem to recognise the (empty) calendars of the other email accounts on my phone (1 IMAP and 3 gmail).

This is a major issue for me. Can anyone help?