It's still probably the firewall.
Good old JimH right again!
Thanks to all who suggested, I took a deep breath and checked the firewall from top to bottom. There is no anti-virus other than Windows. Every entry (on both PCs) firewall reviewed. There are 5 MC entries: a pair for MC23, MC24 for public/private network, an entry for MC 'Extra' whatever that is. All were turn on before. But I figured why not just turn off the firewall and see. Right away DLNA works for both MC23, MC24. So I turned on firewall again without change and both MC23, MC24 STILL WORK! But they didn't work before!
My conclusion: YOU MUST RESET THE FIREWALL AFTER MC24 INSTALLATION BY TURNING IT OFF THEN ON.
I think this strange problem appeared because I upgraded to MC24 soon after Windows 10 itself was updated with major changes. This is the update where Microsoft took out HomeGroup networking, which made all my Win10 PCs cannot share drive/folder anymore. No amount of work can fix it. This, along with breaking MC24 DLNA, is certainly the most stupid Windows update in a decade.