Taken from reddit: https://www.reddit.com/r/MonsterHunter/comments/96h8w5/a_potential_solution_for_constant_disconnections/

TL;DR: add -nofriendsui -udp OR -nofriendsui -tcp as Steam launch options

This will force Steam to use old Friends UI and UDP/TCP protocol instead of WebSocket.

Here's how:

Right click on Steam shortcut (create one if you don't have it), select Properties;

In Target field append " -nofriendsui -udp" or " -nofriendsui -tcp" (w/o quotation marks, be sure to add a space after the existing quotation mark);

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

Exit Steam, then launch it with the modified shortcut.

EDIT: if you are having trouble logging in to Steam with -tcp switch, try deleting (Steam)\config\config.vdf and try again.

EDIT2: according to our experiment the -udp switch works far better than -tcp, but YMMV. Apparently some ISP performs better than the others. Also switching to wired connection can help.

EDIT3: /u/Xavr0k mentioned a registry edit that makes the launch options apply on startup (so you don't have to exit Steam and run from the shortcut). Kudos to him.

==================

Here's how this works (as translated from the op. Credit wu360463231 in Steamcn forum):

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

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

Error code 83: you are disconnected from Steam (Valve) service. (n.b. this is the case when you see bunch of Steam popups that your friends "started" playing games at once)

Error code 5038f: P2P host are d/c or is experiencing interruptions

If the host are d/c, all members in his/her server will be d/c and see 5038f error.

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

Add -nofriendsui -udp

OR

-nofriendsui -tcp

as launch argument of Steam.exe

Which force Steam to use old Friends UI with UDP or TCP protocol, respectively.