I have a leaking Grohe model 20.610 bathroom faucet. I have tried to remove the cartridge, but have been unsuccessful in unscrewing it out. The bolt does not move at all. Am I missing something?

Also, does anyone know if I just need a new O ring or do I need a new cartridge, and if so, what is the cartridge part number? The Grohe site does not have any info on the 20.610 (too old?).