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We have the same bug for co-op game since Patch 4.
When we load an old save (before Patch 4), the game works fine.
However, if we load any save made after the new update, the host machine (either Mac or PC) can load the game just fine. But my wife (on Mac) will not be able to join most of the time - the loading screen would be stuck at 100% and the game just freeze. The rare chance that she can join in, the game would fail to load most of the UI components, and characters T-posing.
We haven't been able to play the game for more than a week now. It's very frustrating. I hope Larian is addressing this issue soon.
We are in Act 3, not sure if it is of an importance.
