Nginx 404 not found/503 error in subdir

Okay, i setup a debian install with ispconfig3 ,php5-fpm and nginx, the
main site, drupal is working fine, even perfect. But i also have a
modded phpbb3 board, in a subdir, named ‘forum’.

Now for some reason i get the following error from nginx when accesing
the dir:

‘ERROR 500 - Internal Server Error!’

This is the content of the nginx directiv field(for vhost):

I checked the php and nginx logs, nothing in it that suggest an error
with php…

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