I initially did have Z set up in vPilot as the PTT key but not anymore.
That was probably the issue-
I also went completely through all the key assignments in Prepar3D v3 and only found the auto pilot master assigned to Z. I deleted that assignment.
That assignment is not relevant, because it's not used in Slew mode, and the editor is working in Slew mode. Check the Slew mode assignments. Maybe, the 3rd party software that took the Z key, has also removed the default assignment for it so, it wasn't enough to remove it, but you also had to reinstate the default assignment.
Try to RESET all your controls to default.