If it is different it is probably your old season ticket number, As it said you already have an account when you tried to activate, I would say you have another account created somewhere that you need to access, try logging in using different e-mail address's than the one you are currently signed into, that is if you have another e-mail address that you may have used in the past?
Like you said, this is nothing ticket office can not sort out, when it is quieter, until then i would just purchase your man city tickets over the phone.