Error message on when someone trying to book their appointment

Same problem, As a service we run around 10 1:N services (on separate calendars) I've checked every setting and everything is the same across the services (although none were changed previously and it worked for the last year or so). What's bizarre is it's only affecting 3 of those 10 calendars, with the others working still. I've tried changing times to no avail, but 2 of the services are mirrored in all settings but the name, 1 works, 1 doesn't. I've tried re-building a couple from scratch and get the same error, I've tried other suggestions from other threads and help articles I've seen many bugs with bookings over the years, but I can't get my head around this one. These calendars are used for training NHS staff and we're getting a lot of frustrated customers.