Problem with reCAPTCHA 2

[GeoffGregory]

I am using WYSIWYG 15 (64 bit).
I have created a comment form which works fine - until I add the reCAPTCHA 2 facility. I have copied the box from the extensions and entered the two Google keys.
When I test he form, all appears well, and the reCAPTCHA box returns the green tick. However, when I click submit I am directed to the error page.
I have tried identical entries with and without reCAPTCHA.
Am I mising something?

[Pablo]

First note that this extension does not come with support. It is provided "AS IS'.
However, there are no known issues otherwise they would have already been fixed.

These are requirements for reCAPTCHA (not specific for the extension):
- PHP 7
- "allow_url_fopen" should be enabled on the server.