I noticed a lot of issues with this plugin and the sudden inability to make any bookings, since around November 20th. I had the same issue and even a complete plugin uninstall did not solve it.
(In my case at least) it turned out to be the "Shared Resources" add-on. It appeared to have gotten totally messed up, i.e. I was no longer able to change its individual service settings. Re-enabling the add-on would not make a change.
The solution was to manually delete in the database table "(wp_)options" the content of "option_value" where "option_name" is "appointments_services_shared_resources". After that I could set the shared resources parameters for each service again.
There seem to be some other issues with it though, in general. For example, any change made to a service's add-on feature (no matter "Location", "Shared resources" etc) it has to be made *twice*: first time it won't save anything, second time it saves correctly.
And it is terribly slow at only 42 services, 27 providers and a couple shared resource settings.