@girish In terms of device we have noticed that it effects phones more. Also, we noticed that if it doesn't work on the phone through data, when we use a phone hotspot it hasn't worked on any device that uses that hotspot - so it seems to be a combination of connection method and device. So far the browser used has had no effect, if it doesn't work on one browser on a given device for a given connection it won't work on all the browsers for that device, although we have less examples of guys testing multiple browsers on different working devices.
Finally I noticed that the problem started almost exactly three months after we set up the americankulak.org domain on the box so it might be a problem with the Cloudron autorenewal for americankulak.org as the other site (which was set up weeks later) survey.americankulak.org is working fine. Although this doesn't explain why the admin page my.americankulak.org has always worked.