Directory index forbidden

Directory index forbidden by rule: /var/www/vhosts/nederdev.nl/subdomains/api/httpdocs/api/, referer: http://api.nederdev.nl/

This is what I get when clicking one of the branch tabs, except for the default one. /var/www/vhosts/nederdev.nl/subdomains/api/httpdocs/ is my Drupal root, alle indexes files are located in /var/www/vhosts/nederdev.nl/subdomains/api/httpdocs/api/[BRANCH]/ with [BRANCH] being one of the branches set up using the API settings page. Indexing files goes without trouble, it's just http://nederdev/api/[SHORT-NAME] that causes a WSOD and the aforementioned error to appear in my logs.

I'm not sure whether this is caused by something on my server or if it's API.module. To me it looks like the latter, because I don't see why the module would need a directory index. Next to that I can't see why directory indices would be forbidden, since API perfectly indexes all files.