Calendar not communicating with exchange server

For the past few weeks (I think) one of my several exchange server based email/calendar accounts is not synching on my two El Capitano macs. The other one is?

The account in question is synching on my two iOS devices. I've removed and reinstalled the account several times, restarting before reinstalling. The accounts have worked fine for the past 2 months. Any ideas out there? thx.