Thanks for the report, ferchault!
Something went likley wrong in this game, as it can't be reloaded (internal error). It's a 3 player game, but from your description the problem doesn't involve 3 players, just A and B. I just tested that scenario in two player games, with and without the Golem and in both scenarios it worked:
Player A is controlling the turn of player B. They make B play Militia, either directly or via Golem. In both cases the list of player A's hand cards is shown on screen for player A, and the appropriate number of which need to be selected to be discarded for Militia to continue.
I also made a three player game, with C the "uninvolved" player. Again A possessed B and made them play Militia via Golem. Of course, now player C is the first one that needs to discard cards for Milita. If they take their time, the game may appear to be stuck. In my test, once C discarded, again A was given the list of their hand cards and had to discard, then the game continued as it should.
Could it be that in your game player C simply took a long time to think what they want to discard?
But then again, as the game doesn't load, it indicates that there was some problem with the game, which now is hard to find.