Found the problem, it was two little lines of code located in my .htaccess file.
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/supercache/%{SERVER_NAME}/$1/index.html.gz -f
RewriteRule ^(.*) "/wp-content/cache/supercache/%{SERVER_NAME}/$1/index.html.gz" [L]
Removed those and everything works fine now.