I found the solution – you can download my modified plugin here:
https://mike.shannonandmike.net/2007/12/18/post-levels-111-fix/
If you are curious, the problem was due to a regular expression that was not expecting the SQL “FROM” clause to use an alias for the “wp_posts” table. My solution, which is a bit hacky, was to insert expressions to create and then properly handle this alias. I wish a better solution exists, but to my knowledge WordPress doesn’t expose these SQL statements through hooks.
