I had the same issue with net::ERR_SSL_PROTOCOL_ERROR when moving from local HTTP tests to a client hosted online. The error usually comes up because the remote client is trying to connect over HTTPS, but the local server doesn’t have a valid SSL certificate or isn’t configured properly. Make sure the forwarded port points to the right service and that the server has a trusted SSL cert (self-signed often fails on browsers). You may need to install a valid certificate from a CA and check TLS version support. Once I added a proper cert and adjusted the appliance forwarding, the SSL connection worked fine. Hope it helps!
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