Form that uses PHP works in preview, not when published

This is an extension to my thread about Post Office, but I’m starting to one to attract a wider range of replies.

I have a form on a site that sends a subscription request to Sendy. It works in preview in Chrome, but not when published.

We are trying to track it down, but not having much luck.

Given the weirdness of working in preview but not on the server, anyone any ideas where we should be looking?

Server logs? Difference in PHP version?