I've been complaining about this one for 8+ years now and it just doesn't seem to be going away.

The most frustrating part is that it doesn't have anything to do with any connections/routing or the servers. It cannot and would not be fixed by any of the additional hardware they've said they need to get but couldn't. It's just the way the client is written to completely close down when that error is encountered, and it should be fairly simple to just kick you back to the title screen to select "Start" again instead. This would also make the whole "tens of seconds" grace period for the lobby server actually useful.

In fact, a friend of mine recently reminded me that if you lose connection normally from within the game (i.e. if your connection to the Internet is down), you get kicked back to the title screen. From there, if you select "Start" while the connection is still down the client closes, but if you wait until the connection is good at the title screen, you can hit Start and get back in without the client closing and having to log back in all over again. All of the pieces are clearly there, it's just badly designed.