error - this staff member's calendar can't be synced.

Hello

 

Booking is a great tool and has been working well for some time

 

but we just started getting an error

 

Error is - This staff member's calendar can't be synced. Reason: There is a syncing issue we can't identify."

 

have removed user, re-added user, same result

 

in the main bookings calendar, it can see all the users including the one not "syncing"

 

any ideas?