Just found a bug in the plugin using cubepoints as payment. I'm also using Appointments+.
The problem occurs only when a new user registers in the site and the cubepoint is set to give free credits to new signups.
In my case i give 5 points to new users (this represents the price for 1 appointment.)
If the user uses this credits to make the appointment, the process seems to work perfect, but in the backend, the status of the A+ stays as PENDING and after a few minutes is canceled (is set to remove after 10min).
According to the A+ developer, that is not related to Appointments, but it's probably related to how MarketPress works with CubePoints.
This happens only with points given automatically for new registrants. If ia add points manually to the user, the purchase happen normally.
Please check what's the difference between these free points and manually added ones.
The former topic for this issue is here if would like to take a look. URL: https://premium.wpmudev.org/forums/topic/bug-in-appointment-cubepoints#post-320596