Error Code: NL-NETWORK-01
Message: Network error: An IO error occurred sending the request.
Details: java.io.IOException: Connection closed during ALPN negotiation.
Could you confirm that you get that error with NeoLoad 5.3.1 and not 5.3.0. This issue should have been fixed in 5.3.1.
It's related to HTTP/2. On the check virtual user panel, if you hit the "Advanced" button and select the browser settings you can disable HTTP/2.