What error do you see in the log or Tracer?

When did you see the error? Was the error shown up when you tried to do private checkout? In that case, the private check out should fail.

When you say that "I am unable to remove this rule from my private checkout rule list", it looks that you successfully private checked out the rule. Then later on, when you tried to open / access it, the error came up. Is that right? If this is the case, did you change your access group or your application rulesets, etc..?

Either one of the above, please check the tracer log and PegaRULEs log, you will see where the error comes from.