Depending on what specific version of v1.8 your running its going on 10+ years old without patches. Running this old version on the public internet is an extreamley precarious situation, and I would urge you to upgrade as soon as possible.
PWM configuration is contained entirely in the PWM configuration XML file. We generally maintain backward compatibility with older file versions, but there is no guarantee - you will need to try it. You may be better off starting with a new config and have the old file open to reference as needed.
As for the error your getting, it may or may not be fixed by updating. You will need to look at logs or enable detailed error messages to get more information about the error that is occurring. The most cause of similar errors is LDAP permissions.