Mouse/keyboard not working on secondary computer

Hello,

I'm setting up Multiplicity 4.07 Pro in Seamless mode with one primary and two secondary computers, each with its own monitor. The primary connects to one secondary without any problem, and the mouse and keyboard work on it. The other secondary shows a red icon with an X on the primary's Seamless connections tab, and the mouse and keyboard don't work on it.

My setup:
- Primary: Windows 11, Multiplicity 4.07 Pro
- Secondary 1 (not connecting): Windows 10, Multiplicity 4.07, connected by Ethernet
- Secondary 2 (working): Windows 10, Multiplicity 4.07
- All three computers are on the same home network

What I've already tried on the computer that won't connect:
- Changed the network profile from Public to Private (the Windows Firewall prompt appeared while it was still set to Public)
- Confirmed Stardock Multiplicity 4 is allowed on Private networks under "Allow an app through Windows Firewall"
- Confirmed the encryption key matches the other computers
- Confirmed all three computers run version 4.07
- Restarted the computer and made sure Multiplicity was running
- Removed the computer from the Seamless connections tab on the primary and added it again
- Temporarily turned off Windows Defender Firewall for Private networks (the red X remained)
So I have been as thorough as possible to figure out why the 1st secondary computer is not working.