Hello,
You were correct to look for the IPN information because sometimes there is some error message there. I assume you have already looked at the troubleshooting section at the adapter’s homepage.
Most probably this is due to some firewall. Try disabling Wordfence, disabling your Linux firewall(s) and also contact your hosting provider to see if they are blocking SSL connections with non-standard headers, such as HMAC authentication.
If you do figure out what the problem was, then you can reenable your firewalls and configure them around it.
Please let me know what you find.
kind regards
Alex