I had a similar issue where I was trying to multicast from playercontroller and other clients were getting unpredictable results, like world getting unloaded, and client freezes.
solution for me was to move multicast call to my Character BP.
Another poster said:
“This is infact correct and only solution. Clients only are aware of their own PlayerController and don’t have PlayerController objects for other players. Running a Multicast on a PlayerController causes bad things to happen because you’re telling all clients to run a function on something that technically doesn’t exist. Only Server is aware of all PlayerControllers.”