Sending fails first time, succeeds second time

I have a simple form, it has been working for years. A problem has arisen.

The form fails (with the failure message) the first time you press Submit but succeeds the second time you press Submit.

This is consistent in most browsers, it doesn’t happen in Firefox. There are no console or PHP errors. The page has completely loaded.

What sort of thing could cause this.

The page I need help with: [log in to see the link]