Was getting the same issue on IIS. In our case we had 2 causes:
1. We were blocking admin-ajax.php from the public web
2. curl was failing to validate Amazon/WeTransfer’s certificate
The first one is simple, make admin-ajax.php accessible. This probably won’t be your issue though as most people don’t IP block their wp-admin folder. The second one is the most likely. Your host either has not configured a certificate authority bundle or the one they are using is out of date. You can test this by disabling SSL verify host/verify peer – which is done by editing the OWPWT_CurlService.php file in /wp-content/plugins/ozpital-wetransfer/app/Services/. The ideal solution would be to update the CA bundle.
