The version of auth-ldap last tagged as openacs-5-1-compat is from January 2004. There has been a change in the authentication APIs adding additional parameters.
The lastest auth-ldap should work, but it has not been tagged in over a year and cannot be installed from the repository.
Can a developer more familiar with the package please investigate this?
Please make sure to increase the version number in the info file before tagging so the APM will notice the upgrade and install the new version from the repository.
Thanks