I checked your site and could see the issue, and did some basic debugging steps which didn't reveal anything.
I don't have FTP access to check your debug logs which may have useful info, but as that might mean I would need to take a copy of your site for testing I am taking one anyway, and I'll install locally and check what the logs show.
I'm doing that now and will tidy up when done and then get back to you with what I find.
I installed your site locally and tested the form and it worked fine, so it appears to be a server-related problem (or an issue arising from the must-use plugins which I disabled to use your site locally but which you cannot on WP Engine).
So can you please check your PHP logs?
If you haven't already, turn on the debugging by editing your wp-config.php file and change the line with WP_DEBUG like so:
That will create a debug log called error_log.txt in your root directory (the same location as wp-config.php) which you can examine in any text editor. Try uploading an image to the form again and then inspect the log.
Hi Nigel, thanks for the investigating. We've had a long talk with WP Engine and they could not find any errors.
So after some more hunting, we found the site was switching between http and https which seemed to mess with the CRED form.
We've now forced it to always be https and it seems to be working now.