Can you also please provide the exact nick you are trying to register, the list of staff nicks from /msg OperServ staff and /mode <yournick> please?
Also it should be noted that when you have extremely short OperServ staff nicks the option RestrictOperNicks can cause trouble for anyone with a nick that so much as contains the nick of an OperServ staff member. For example if you have a staff member with the nick "E", then any nick containing "E" will be denied.