Error message when trying to add new user

Hello,

Customer is working on 2005B (2005B 7.40.252 SP00 PL39).

They are working on this database since 2009.

No upgrade was recently done.

when trying to add a new user they get athe following error:

"A user was added outsite the application. Delet it, or preform'Restore Numbering' [Message 3512-7]"

Could you please advice.

Regards,

Maya