WPMU 3.5.1 internal server error

I have just converted a normal WordPress installation into Multisite and a few things aren't working properly.

I receive a 500 error when I try to view the inactive plugins page.

I cannot upload images. After I upload, they are not on the server and I receive a 500 error when trying to go to their url.

I've tried different .htaccess configurations and have set up a few multisite installations in the past.

This is the first time that I've converted a custom WordPress install into a Multisite install. I'm using the normal WordPress file structure and these are the only two errors that I have.

Here is my .htaccess

# WordPress SEO - XML Sitemap Rewrite Fix
RewriteEngine On
RewriteBase /
RewriteRule ^sitemap_index.xml$ /index.php?sitemap=1 [L]
RewriteRule ^locations.kml$ /index.php?sitemap=wpseo_local_kml [L]
RewriteRule ^geo_sitemap.kml$ /index.php?sitemap=wpseo_local [L]
RewriteRule ^([^/]+?)-sitemap([0-9]+)?.xml$ /index.php?sitemap=$1&sitemap_n=$2 [L]
# END WordPress SEO - XML Sitemap Rewrite Fix

# Created by Redirection Module: Apache
# Thu, 14 Feb 2013 09:02:54 +0000
# Redirection 2.2.13 - http://urbangiraffe.com/plugins/redirection/

<Files .htaccess,.svn>
order allow,deny
deny from all
</Files>
Options +FollowSymlinks

<IfModule mod_rewrite.c>
</IfModule>

# End of Redirection

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]