Here is a possible workaround, which worked for me, for anyone who may be struggling with the infinite load screen on the Precious Cargo side quest:

Move the guy's horse to the other side of his camp before you talk to him. Also move Roach just next to the road.

The problem is that the guy tries to get to his horse, but usually players will park Roach right next to the guy's horse. So what happens is that he tries to get to his horse, but he first moves to his right (Geralt's left) which means he now has to turn back around and in his way are the fire place, Geralt, and Roach.
By moving his horse to the other side of the camp his first move to his right is in the direction of his horse, so he can just climb on it.

Try it and see if it works as well for you as for me.