@michaelh: Thank you for the suggestion. I have the same problem when all the plugins are deactivated: the admin panel menu items work as expected EXCEPT for the two cases I mention above.
I also tested the domain settings, as my wp install is domain.com/blog but the blog address I use is domain.com. I still get the 404. The tool menu items point to /blog/wp-admin/tools.php, which is correct. So why the 404?
All other menu items work as expected in the admin panel. Any suggestions are most appreciated.