Whenever a brave Rainbow Six Siege newcomer joins a game with my usual group, we offer a gentle warning: This might take a while. If the game restarts a couple times when you launch it, don't worry: That's normal. Can't see the squad invite? Normal. Some kind of crossplay matchmaking error? Yeah, that's normal. Try restarting the game. Try restarting Ubisoft Connect. Try restarting your PC. You're in? Great. You crashed? Normal...
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Rainbow Six Siege is so consistently buggy that I can't even be mad anymore
Source: PC Gamer (opens in a new tab)