Ian, was that by chance a ProPilot flight? Because if it was, the behaviour is normal: every time one books a flight, the server "knows" how many passengers are waiting for that flight. Tht number of passengers is included in the PIREP abd CANNOT be modified.
In this case, if one is looking for realism he should do the other way around: note the number of passengers given when the flight is booked and use that number in FsPassengers or any other load configurator around.
Ashley, that issue sounds familiar enough, it is related to some DLLs that sometimes need to be registered manually after install. There is extensive history of this problem in this forum, just search for "runtime".
Andrei