Similar issue but triggered on 2nd vassal already.
Force surrender with vassalization not processed correctly (including no trade, no war support reset, no treaty change, etc.)
Existing vassal no longer available with diplomacy (clicking icon no longer respond, and appear with a mutilation sign, possibly due to stability=0 as civ removed?)
End turn and it goes to Turn Pending.
Tried both on steam deck and cloud save on pc, same error.
When I try to vassalize more than 3 civs, the game bugs and messes up all my vassalized civs. Let's say I had blue, green, red and I'm going to vassalize purple. When I force surrender them, the game randomly removes an already vassalized civ and do not add the new one. going to the next turn gives the error. Any other action works.
maybe this? Unknown enum 'DiplomaticStateType' value. (value=VassalToExternal)