Can't write new files, but can modify existing

I've just set up a dokuwiki page on an apache2 server running on Ubuntu 16.04. When the dokuwiki tries to create a new page, I get a http 500 error (The IP page isn’t working. IP is currently unable to handle this request.). However, I can edit and save an existing page without problems.

I've looked at the list of things to fix https://www.dokuwiki.org/faq:error500, but I'm not sure where to start. I'm not doing any pretty URL rewriting that I'm aware of. My .htaccess files look like:

<IfModule mod_authz_host>    Require all denied</IfModule><IfModule !mod_authz_host>    Order allow,deny    Deny from all</IfModule>

My bin, data, inc, lib, vendor, and conf folders are all drwxrwxr-x and owned by www-data.

Can someone please point me in the write direction?