The following SQL code does not detect that a user is an Administrator and denies access:

  my $query = "SELECT 
                 hashed_password, auth_source_id, permissions
              FROM members, projects, users, roles, member_roles
              WHERE 
                projects.id=members.project_id
                AND member_roles.member_id=members.id
                AND users.id=members.user_id 
                AND roles.id=member_roles.role_id
                AND users.status=1 
                AND login=? 
                AND identifier=? ";

Adding myself as a member of the project within redmine permitted access.

Note there is a related issue: #3712, where someone has uploaded an "improved version" of Redmine.pm, having significantly enhanced features. I will try this version next, to see if it allows Redmine administrators to have access to the Redmine-managed subversion repositories. Perhaps it would be worth accepting the improved version into the main distribution as an easy fix for this issue (if it works)?