During the next turn Asoka comes and asks me to declare war on the Germans. I click yes and then the game crashes. I think that this is because I have 6 vassals and the game can't cope with so many simultaneous war declarations. Just click end turn on the attached save file and you'll see what I mean.

I suspect I have the same problem. I'm playing as Qin on the Earth18civs map and have a defensive pact with Asoka, and have vassalized Frederick. Upon declaring war on the Persians, who have the Spanish as their vassal, the game freezes.

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I think I've also solved the problem (for my own save game at least).

I had a whole bunch of Privateers that were blockading different ports. Upon declaration of war, a whole lot of trade routes had to be recalculated which made it (appear to) freeze. Solution was to temporarily unblock all ports, then declare war, then blockade everything again. It's slightly annoying since I have 8 or 9 harbors blockaded.

I had a whole bunch of Privateers that were blockading different ports. Upon declaration of war, a whole lot of trade routes had to be recalculated which made it (appear to) freeze.
The only game I've used blockade, it looked like the game was hanging, but after a handful of minutes I could play on.