ssl - What is NSS error -5961 (PR_CONNECT_RESET_ERROR)

Can someone enlighten me the meaning of "NSS error -5961 (PR_CONNECT_RESET_ERROR)"?

I am trying to connect to bitbucket.org with "https" protocol but got a refuse from the server. Then, I try to use curl on the command line and see this output.

# curl -v https://bitbucket.org
* About to connect() to bitbucket.org port 443 (#0)
*   Trying 131.103.20.168...
* Connected to bitbucket.org (131.103.20.168) port 443 (#0)
* Initializing NSS with certpath: sql:/etc/pki/nssdb
*   CAfile: /etc/pki/tls/certs/ca-bundle.crt
  CApath: none
* NSS error -5961 (PR_CONNECT_RESET_ERROR)
* TCP connection reset by peer
* Closing connection 0
curl: (35) TCP connection reset by peer

With openssl, I got this output.

# openssl s_client -connect bitbucket.org:443 -msg
CONNECTED(00000003)
>>> TLS 1.2 Handshake [length 00f4], ClientHello
    01 00 00 f0 03 03 55 59 80 fa 72 25 f4 a5 84 49
... 
write:errno=104
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 0 bytes and written 249 bytes
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
---

Answer

TCP connection reset by peer

This error from NSS is the same error you get with openssl (errno=104: ECONNRESET). This simply means, that the peer or some middlebox in between (firewall) is terminating the connection.

Since the site is reachable from my place I would suggest, that there is a firewall on your site blocking the connection. The behavior is fairly typical for DPI firewalls in that the initial TCP connection is allowed but once you send the first data (ClientHello from TLS handshake) it will determine if your access is allowed by policy and let it pass or deny it by injecting a TCP RST.