Strange, may be it's related to ssl configuration of NB or browser.
Can you pls. monitor the communication of port 4848 using some network
utility (like wireshark) and send the snapshot. Cause now i'm not sure
at which step communication get stuck.
According to server.log message it's peer (browser or NB) close the
connection, may be it's due to some security reasons.
Thanks.
WBR,
Alexey.