Hrqls: I'm not really sure, as I never worked directly with Serial Port. But parity errors normally occur if
a) there occured an error during sending and one bit was changed or
b) maybe the parity bit was corrupted by your program?
Maybe your connection is set to a bytelength of 7 bit and you overwrite the parity bit? Again: I don't really have a clue, but this was what came to my mind ;) Good luck in finding the solution :)