When upgrading from Application Server 8.0PE to 9.1, an error is thrown saying the server does not have system connector named null, and invalid user information as seen in sbs-manual. Even after changing the hardcoded values, the same error message is seen. This occurs because the domain.xml has changed between 8.0 and 9.1.
You can only encounter this bug while upgrading from a 8.0 PE to 9.1. The workaround is to upgrade to either 8.1, 8.2, or 9.0 and then upgrade to 9.1.