So reaching this assertion seems to mean: there is no revision for this package version, and we have not noticed any later versions imported from debian, and there is a 'mark' for this revision in the revids db.
There is a revids entry for this
openldap|2:18.104.22.168-1|woody|<email address hidden>|e4856fefd49b5718673d07bd9e1f675f119d4a00
So why is it marked but not imported? Perhaps this is no longer present in the branch.
It seems the problem is due to the quirky (illegal, but factual) order of revisions of openldap, and that the importer checks for debian imports as it goes along. We can probably handle this by scanning for debian versions before making any decisions, rather than checking as we go along.