Constant Contact: ERROR: Failed to establish connection

Does this Constant Contact form integration still not work with PHP 7.x – the version WordPress recommends running?

I’ve followed the instructions found in the docs, https://contactform7.com/constant-contact-integration/

After creating the app obtaining API key and App secret, and trying to make the connection, I receive this error.

ERROR: Failed to establish connection. Please double-check your configuration.

I’ve tried signing into my Constant Contact account first, but it doesn’t matter. The error persists.

Tried re-creating an APP secret 3 different times, still error persists.

Tried using different PHP versions – PHP v7.2, v7.3, v7.4.