WordPress redirecting legitimate directories

I have WP installed in a domain root, along with a whole bunch of other stuff in other directories, and accessing these subdirectories has never been a problem.

However, I recently installed phplist (https://www.phplist.com/) and I cannot access https://www.mydomain.com/lists/ or https://www.mydomain.com/lists/admin/ (req’d for setup) – all I see is the WP 404 page. Both dirs have index.php files, but I can’t access them directly either (e.g. https://www.mydomain.com/lists/index.php).

I was using 2.1, but updated to 2.5.1 and still have the same problems. I am not password-protecting anything with WP, so this is not the usual WP directory issue. I’m using the standard pretty permalinks htaccess.

All help greatly appreciated.