Have been playing Ultima for over about a year, but have only had this happen in the last month. Sometimes whenever I use the shortcut "/lobby" when returning to the lobby, the game will simply close. No error message or notification that the app has crashed, game window just disappears. It usually happens two or three times an hour if I'm first-room farming and jumping back to the lobby frequently.
This has also just been happening at random, without using /lobby, though less frequently. Can be idling or in the middle of a fight, the game just closes. My internet is fine during the time, so I know it's not a disconnect (I at least get an error message for that)
Not sure what is causing it, this didn't use to happen.