In the cases I've looked into that's not the problem. A connection to the lobby server stays alive the whole time during pregame, as well as during the game itself. Listing possible cases doesn't help. I'm interested in the actual cases which result in that problem.

It's caused by a delay in the lobby bots before it can confirm that a player has quitted. This is common when there are many players joining and leaving at the same time On the client side, the gui has already reached the pregame page but a backend connection might still be alive - bad connection to host, especially intercontinental games - game bug or crash, e.g. suddenly cannot click, OOS, RAM overflow - OS or hardware failure - player is distracted by real life situations

There is no release version of a27, I suppose you mean main branch. If an identical error is printed you either didn't build what you thought you did or didn't run what you built. This exact error message does no longer exist in main. Building a26 at this point from a checkout is non trivial, I suggest to inform the maintainer for the package for your distribution (usually bug report) and if you want to have a go at it yourself use the package build scripts, which have all prior fixes, and the package build tools of your distribution to create an installable package instead.   You can't catch a segfault.

The cases I looked into so far were all caused by being online with multiple instances of 0ad. So unless that was done intentionally it'd be interesting to figure out why that happened. I believe it might be important to learn why rejoining a running game is necessary in the first place. Is that because 0ad crashes or because the player looses the connection to the host?

Link says 500 BC to 500 AD I can work with that!

So its 500bc to "0ad" with some fairly lenient exceptions, like Boudica.

It is explained here https://play0ad.com/game-info/project-overview/