All players connected to the same local router (preferably via Ethernet). Mod playlists synchronized and verified across launchers. Game speed locked to 3 or 4 to eliminate desync risks.
An Out of Sync error happens when one player's PC processes game events differently than the host's PC. ck3 lan multiplayer
Although Paradox has improved their servers, hosting a game on a LAN (Local Area Network) offers several distinct advantages over traditional internet multiplayer: All players connected to the same local router