Hi ,
I have a project setup that I think is close to what you have when you see this crash occur. I have two sockets on a player character, and on begin play I attach an instance of an actor class to each socket using the AttachToCompSocket code you provided above. Unfortunately I did not see a crash when I tested this in a few different multiplayer tests, so I am still missing something. Would it be possible to see the entire code for your character class and the class you are attaching to the character?