Does your RFC call always fail when executed from the Personas script? Or does it work for the very first time and then it fails when you call it again and pass changed parameters to it?
If it always fails from Personas, then I have a hunch... do you actually have any value that the FM returns to Personas? If you don't (so you only pass something but there is no return in table htmldoc) then that may be the reason. My experience is that Personas wants to receive some result back, even if it is just a dummy variable that won't be used. At least that's what I found... I may be wrong of course.