Whilst performing an upgrade through the admin panel, the site locked. I had to manually delete the .maintenance file to get it to work.
Since then, I have been unable to save any articles/pages. It goes to the correct url (/wp-admin/page.php) but returns a 404. The same happens with post.php.
To try and repair this, I regenerated the htaccess, which didn't help. When I remove the htaccess, it changes to an error 500. This leads me to believe it is actually 500 error, but there is no handling page for 500s, so it returns a 404 when looking for that error page.
I reinstalled wordpress from SVN, reinstalled the skin, and generated a new (simpler) htaccess, but this did exactly the same. I even added a line into the htaccess to exclude /wp-admin/ from rewriting, but this didn't have any effect: rewriteRule ^(wp-admin) - [L]