You had real race 2 with that other account. For Real flight 3.5 you will have to contact support for any help with the serial # issue. I assume you recently aquired RF, if that is the case, contact the person you purchased RF from or the KE support @ 217-398-8970 (Option 1) (Available Monday-Friday, 8:00 AM - 7:00 PM CST.)
Good luck.