Game crashes on repeated start attempts
What is the freeciv version number where you encounter this?
I'm not sure what you mean by "Experimental" and "Mainstream" versions. Stable vs Development version, or maybe freeciv-2.6 Windows builds with msys1 vs msys2, or something completely different?
I couldn't easily reproduce such problem in any active branch (on Linux). The menu disappears as soon as I start the game. It's possible that menu disappearance (or menu getting disabled on other clients) has been implemented so that it's not immediate, but only gets triggered by, e.g., response from the launched server, and could then happen too slowly if it takes long to launch that server. In such cases we should just ignore new attempts to launch the game if it's already ongoing - I have to check if that part has some problem.
Can't reproduce, at least without more information about version and environment.
When attempting to start multiple games very quickly, the game crashes. This happens only in Experimental-version and not in Mainstream-version, because in the Mainstream-version option to start a game disappears when a game is starting.