CF7 v5.1.3 fails to send using WordPress 5.2.1

Hi,
I updated to WordPress 5.2.1, and updated my plugins and themes. I also updated the server to use PHP 7.2. I then tested my contact form, and it failed to send (red border). I tried again (as sometimes I’ve had it fail the first time) and it failed again, in multiple browsers, either with an orange or a red border.

I tried disabling the reCaptcha (I’m using v3 integration), and it still fails, in the same way.

I tried switching to the Twenty Seventeen theme, and it still failed.

I tried disabling most of my plugins (everything but Flamingo, WP Mail SMTP and CF7) and it still fails.

I tried downgrading PHP to 7.1. No change.

Interestingly, it is saving the message to Flamingo, and it shows that it passes the reCaptcha test.

The server error logs show nothing important.

There are no errors shown in the console.

The WP Mail SMTP plugin has a send test message feature, and it is working correctly.

Can anyone give me ideas of other things to try?

Thanks!

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