I tried using PHD2 natively to guide the AP900 and I get the same result:  the AP900 is tracking fine until I initiate PHD2 calibration or guiding.   Within 2-3 seconds of initiating PHD2, the tracking on the AP900 ceases.
After halting PHD2, I can re-initiate tracking on the AP900 handset, but every time I try to get PHD2 guiding  started, it stops the AP900 tracking function.

Is there some setting or other that I'm not getting correct?

thanks!