That's maybe just another Microchip way of reinvent the wheel.... This may be another specific device who need another special method to write to eeprom and plah plah... BUT how about if you use the assembler method stated in the datasheet AND/OR errata to write to the eeprom?

Could also be another MPASM problem... wich is still possible. Did you contact Microchip tech-Support http://support.microchip.com Usually they answer within 24 hours

Sorry Alain i don't have any handy 18F1320 to test it. I'll order some sample for myself too to play with them when health permit to stay awake more than 1 hour steady.