If the nextcloud install is in a subdirectory on the webserver, such as https://serverexample.com/nextcloud/, this fails because templates/iframe.php refers to all of its assets by absolute path as /apps/ocs_api_viewer/, which don't exist.
I corrected this on my install by modifying those URLs to instead be ../, but refrained from putting in a pull request, because I assume that there's a more correct way to do that which I'm not familiar with.
If the nextcloud install is in a subdirectory on the webserver, such as https://serverexample.com/nextcloud/, this fails because templates/iframe.php refers to all of its assets by absolute path as /apps/ocs_api_viewer/, which don't exist., but refrained from putting in a pull request, because I assume that there's a more correct way to do that which I'm not familiar with.
I corrected this on my install by modifying those URLs to instead be ../