Resolution
When importing a Rule into the system that is fully functional and has previously worked, a user may experience an error when trying to view the Rule with the "..." button of the Rule Editor: "an unexpected error occurred: Rule Editor configuration error."
The error message is misleading and is not definitive. The actual problem is that the rule is missing a type declaration, a requirement since Access Governance Suite 4.0. It is possible to write a Rule in Beanshell that is fully functional that simply lacks the type declaration and will give this error. Check the Rule in the debug page and verify that the Rule Type is being declared within the Rule. If it is not delcared, you may either add this into the Rule within the Debug page view and save or add the type and reimport the Rule as desired.
This was fixed in Access Governance Suite 5.0p6 and later, but the workaround is to simply add the type declaration to the Rule and the problem is resolved.