Does your even “Event Cast Spell” also need to be called on the server?
Ive had an issue in the past where I was getting the player, but the client player, not the server. So what happened was I was getting old or null variables. What eventually needed to be done was to call all events that needed a reference to the player on the server and this seemed to fix my issue.
If that doesn’t work, what I suggest would be to print out the results of each time to get or set the aim transform variable, and see where the issue is lying at what stage in the process. It sounds like the underlying code behind what you are trying to do works, however getting that back when spawning fire ball still has a few hiccups.