The September event, appeared, without text, in my turn resumee (bottom of the screen).
I went to the F3 screen (military screen) and saw the option "September convention": after clicking it,
I had 2 options to click: either "sign" or "refuse to sign".
I've chosen "sign", closed the screen with the cross on the upper-right hand of the box, and resumed my turn.
Next turn, nothing happenned (in the resumee) and the F3 screen had exactly the same option.
Did I miss the right time to sign it?
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btw, concerning low contentment, I finally managed to raise it all to very acceptable levels (70% or more), although it
had gone very very low, with strikes & riots in several provinces. Maybe a bug has been fixed this summer?