RHEL5: TCP connection to a specific TCP port fails

Issue

• This RHEL5 client fails to generate a SYN packet only when it tries to connect to any remote IP at one specific TCP port number, 9xxx.
• Connections to 127.0.0.1 or the local client IP address at port 9xxx work fine.
• netstat/ss show the socket in SYN_SENT state, but no SYN packet is ever seen on a tcpdump taken locally with -i any.
• Connections to any other port number work perfectly fine!
• No iptables rules. We unloaded all their netfilter modules and it did not resolve the issue.
• SELinux is disabled
• No obvious error seen in netstat -s