Please do a full **clean** re-install:
- move any valued save games elsewhere
- "delete local content" in Steam-Stellaris
- manually delete both the Steam/SteamApps/common/Stellaris AND Documents/Paradox/Stellaris folders
- re-install game in Steam, run a Steam Verify when done.
- start the game with no mods active and test
- make sure all players have opened the required Steam ports in all their active firewalls (each player may have up to 3)
https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711
- use no mods at all
- all players use the same language in-game
- when anyone gets an OOS error, re-host the game immediately, as it will almost certainly crash soon
- start your multiplayer games and wait half a minute or so before unpausing without any player performing any action at all. Only after the game has been unpaused should players begin doing actions like setting research, exploring etc.
Good luck!