This tripped me up the other day too - it's annoying because it would not have been hard for IBM to put a friendlier message on the screen.
In my case I was trying to run the task on a server that was not the Admin server for the domain - would be an easy error to point out to a user
Also setting debug_autopop=2
Gives loads of useful information, but only if you actually get Autopop to run, so check step 1 again.
Lastly as autopop is built as the result of an LDAP query, wouldn't it be great if we could just add our own queries into the group definition?