Gallery is only showing a JSON with supported file types

When I open the gallery app on my Nextcloud instance (11.0.2.7), instead of showing me the gallery page it is showing this:

{“features”:[],“mediatypes”:[“image/png”,“image/jpeg”,“image/gif”,“image/x-xbitmap”,“image/bmp”]}

And nothing more, I have no gallery.cnf or whatsoever and in php/nginx logs are no errors.
Maybe somebody here can help me out.
This Nextcloud instance is running on a Raspberry Pi with Ubuntu 16.04.