Multiple Appointments aren't allowed on A+

I picked a service from the drop-down menu
I clicked on a date on the calendar, then picked a time.
I then proceed to choose a different service from the drop-down menu.

The screen refreshes, but it NOW does NOT let me choose any free time, even though they are all available. What I see is a little, red circle with a cross on it and my mouse is unable to choose a time slot.

I even tried to confirm the first appointment, before making payment, and it still doesn't allow me to add a second appointment to my "shopping cart". I'm using the product theme.

It does, however, show up on the pending appointments in the back-end.