Unable to schedule meetings

Hi

All of a sudden none of our desktop PC or Mac clients can schedule meetings in the Zoom app.  On PCs the save button is greyed out, and on Macs, the save button is greyed out and the warning " Some fields require your attention" is displayed.

Scheduling with the Outlook plugin works fine. Calendar is syncing fine to Office 365 also. 

Any ideas where to start with this ?

Many thanks

Ohgr