After looking at the problem described here http://forum.shuffleit.nl/index.php?topic=1762.0
some more, I formulated a theory about what happened and tried to test it. The following results occured, that show incorrect behaviour in some very constructed situations:
1) Game #3156638: I have played Crown with Horn of Plenty (situation at end of game). I can gain cards costing up to 4 coins, which includes Villa and Peddler that costs 0 at the moment. If I choose Villa for the first playing of Horn of Plenty, the Villa ends up in my hand, my Action count has increased, so all indications show that the Villa trigger ability has occured. But it is (internally) still my buy phase, as can be witnessed by the fact that Peddler still costs 0 and can be gained with the the second playing of Horn of Plenty. If that is done, the game *now* returns to the Action phase, allows me to play Actions and shows Peddler as costing 8 again.
2) Game 3167909: I have played Crown1, doubling Crown2, which doubles (the first time) a Horn of Plenty (situation at end of game). Again I can gain a Villa and some other card (doesn't matter) for the second playing of Horn of Plenty. Again, the Villa ends up in my hand, my action count increases by one, so the triggered ability of Villa has happened. But again the game is internally still in the buy phase, as can be seen by the second playing of Crown2. It should now be my Action phase, this second playing of Crown2 should allow me to double an Action. But instead it allows me to double my treasure card, as though we are still in the buy phase.
It seems that these problems as well as the one described in the other thread are caused by a slight delay of returning to the Action phase, which normally doesn't matter but can in some (admittedly very constructed) circumstances.