How can I troubleshoot my router NOT routing between LAN PCs?

I have a FiOS router, which has 3 PCs connected to it.

One of the PCs used to run several servers/services; that I previously (using a different router from different ISP) successfully connected to in the past: e.g. FTP server on port 2222, RDP on 3389.

Now, when I try to connect to that PC from other PCs on a new router, NONE of the connections work.

PC in question is Windows XP without any firewall software, IP is allocated using DHCP, router is Verizon FiOS's Actiontech MI424WR-GEN3I.

Here's what I tried:

• The PC in question connects to Internet successfully (e.g. web browser, chat, etc.. all work)
• The PC in question is visible from the router's network device list, and the IP matches up with the IP shown via ipconfig /all on the PC
• The FTP server on the PC in question managed to successfully add itself to port forwarding tables on the router, I see the following rule added: Serv-U_Auto TCP Any -> 2222
• I tried running tracert to that PC from another PC. Says "Request Timed Out"
  • tracert to the router itself works in 1 hop.
  • tracert from the affected PC to other PCs also times out.
• I tried telnetting directly to open ports (2222 and 3389). Times out.
• I even tried ICMP ping test connection feature of the router itself to that PC. That timed out as well, 100% packet loss.

What else can I do to troubleshoot this inability to connect to the PC?

It looks to me as if the router simply stopped routing between the PCs on a LAN, without affecting routing to the WAN internet connection.

But I can't figure out how to verify that in the router's web config software.