So I’ve solved the problem. It was a firewall problem on the PI.
I was not aware that iptables was enabled and its outgoing rules did not allow connection on port 1200.
I diagnosed the issue by using tcpdump to catch all packets to port 1200 and discovered it did not even see the first TCP handshake packet. That made me suspect the packet was dropped at the first place.
After I cleaned the iptables rules, everything back to normal, I was able to claim my new feed.
Many thanks @obj