Access Denied when installing plugins in a multisite

Hello!

From time to time I’m getting Access Denied when installing plugins in my WordPress multisite. This doesn’t happen to all plugins but some. For example these two plugins give me Access Denied error: http://codecanyon.net/item/go-pricing-wordpress-responsive-pricing-tables/3725820 and http://codecanyon.net/item/media-grid-wordpress-responsive-portfolio/2218545

What can cause this?

My .htaccess looks like this:

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