Hello! I haven’t used FSX multiplayer myself, but I did some digging and wanted to help.

The issue you're, describing network errors when trying to create a multiplayer, session is most likely related to network configuration, not the race settings or your internet speed. FSX multiplayer (especially in Steam Edition) is notoriously picky.

Here are a few things to check:

Port Forwarding: FSX needs certain ports open to host sessions:

TCP: 6112–6122

UDP: 27000–27100 If these aren’t forwarded on your router, FSX might not let you start a session.

Firewall or Antivirus: Make sure FSX is allowed through your system’s firewall. Some security tools block multiplayer traffic by default.

Steam Multiplayer Settings: When setting up your session, double-check that "Make session visible to everyone" is selected.

Try a Free Flight Session First: See if you can successfully host a regular flight — this helps determine if the issue is with the Longleat race map or your overall multiplayer setup.

Mission Sync: If you’ve modified the map (like changing it to 4 laps), make sure everyone joining the session has the exact same mission file. FSX requires identical files for mission-based multiplayer.

If things still don’t work, tools like JoinFS or FSHost might give you a smoother multiplayer experience.

Hope that helps!

(Credit to Microsoft Copilot for helping with this answer)