i know, this post is older than a year, but maybe this helps you too. I saw on your website, that the problem persists.
Check out this video (in german). You’ll find the answer from the 6th minute on.
Edit the file nextcloud/lib/public/AppFramework/Http/ContentSecurityPolicy.php.
Find the line contains allowedFrameAncestors. Normally, there is only one next line with '\'self\'',. Under this line, add a new line with the domain name of your website. For example: 'my-pretty-cool-website.net',.
Save and close the file. Reload the site, contains your NC Appointment Form and feel happy.
If you can’t edit the Nextcloud configuration file, may you can ask the responsible administrator to do that for you.