I just found this answer in the WordPress bug Trac. Applying the code update worked for me.
Multisite 3.1.1 Upgrade Redirect Loop Network Admin & Plugins
(modifed the code in the file “admin.php” in the path “/wp-admin/network”)
http://core.trac.wordpress.org/ticket/17090
I installed WP 1.5 this morning successfully. I was able to get the podcast enclosure functionality to work. But, when I would click the ‘Entries (RSS)’ link at the bottom of index.php to go to wp-rss2.php I would get a 404 error. I finally found a google link to the following url that gave me the fix:
http://mosquito.wordpress.org/view.php?id=903
I am using IIS 5.0 and I had to modify wp-blog-header.php. I changed the line
(false === strpos($_SERVER[‘PATH_INFO’], ‘index.php’))
to
(false === strpos($_SERVER[‘PATH_INFO’], ‘.php’))
Dropping ‘index’ from the .php extension in the line fixes the problem. There is an issue with the way the path is returned and this produces an incorrect reference that results in a file not found.
I hope this is of help to someone.
