Just to confirm, I had essentially this same issue with a Prolific RS232 adapter. Using a new adapter with the ftdi chipset solved the problem for me.
While I was waiting for the ftdi adapter, I was able to use the mount over wifi, but only with the RS232 cable disconnected.
Something with the prolific chipset (DTR maybe) kept re-initializing the controller and preparing the propeller for programming just by having it connected. The prolific driver had multiple manual settings (like reverse DTR or disable DTR, etc...), but changing those had no effect.
Mounts: EXOS II PMC-Eight, LXD75
Cameras: ZWO ASI294MCPro, ASI385