• Right click on Steam shortcut (create one if you don't have it), select Properties
  
• In Target field append -nofriendsui -udp or -nofriendsui -tcp (be sure to add a space after the existing quotation mark)
  
• Restart Steam with the modified shortcut.

The final result should look like
"C:\Program Files (x86)\Steam\Steam.exe" -nofriendsui -udp

• Press Windows + R
  
• Type regedit and press enter
  
• Browse to Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
  
• Double-click on Steam
  
• Add your preferred launch options to Value Data and press OK

The final result should look like
"C:\Program Files (x86)\Steam\steam.exe" -nofriendsui -udp -silent
The -silent argument will make Steam start minimized in the system tray.

(posted by wu360463231 in Steamcn forum)

The multiplayer is based on P2SP (peer to server and peer), in which players have to keep connections with other players and also the Valve server.

There are Valve servers in Jiangsu, Hong Kong, Japan, Singapore, LA, and other regions. The server you connect to depends on the host.

Error code 83: You are disconnected from Steam (Valve) service.
The following reconnect causes the spam of Steam popups that your friends "started" playing MHW

Error code 5038f: P2P host is disconnected or experiencing interruptions
If the host is d/c, all members in his/her session will also d/c and see this error.

Solution: Force Steam to use older Friends UI, which is based on an older protocol rather than WebSocket like the new UI.