From time to time, the Dropbox Add-On may incorrectly return a false negative in its checks for an SSL certificate present on the site. Here’s how to fix the issue.
Use the gform_dropbox_ssl_compatibility Hook
The gform_dropbox_ssl_compatibility filter will allow you to override any checks for an SSL certificate that are performed by the Dropbox Add-On. If you’re having issues, but are certain that an SSL certificate is installed correctly, this hook will override the SSL checks; allowing you to continue using the add-on.
add_filter( 'gform_dropbox_ssl_compatibility', '__return_true' );
Placing this code within your active theme’s functions.php or within its own plugin will override any SSL requirement checks within the Dropbox Add-On.