Appointments+ error: we are sorry, but this time slot is no longer available

I have a multisite with several subsites running the same settings using Appointments+, but I'm only getting this issue on one of them.

We have appointments setup with 2 services. One of them has 4 providers and all them have the same schedule, so we have 4 slots available for it.

The issue is that, for some reason, when users click an available appointment slot, they are receiving the following error:

"We are sorry, but this time slot is no longer available. Please refresh the page and try another time slot. Thank you."

The slot is showing as available on the front-end and the slot is also not fully booked when I've checked the appointments in the WordPress admin. So there is one spot left that appears as available but can't be booked by the users.

I tried running a conflict test and the issue is still there. I also checked it when having no add-on active on Appointments+ and the issue persisted.