In addition to this, if you create the “Server Custom Event” on an actor like a Projectile having its own Blueprint, you probably won’t trigger the “Server Custom Event”.
I had this issue, although everything was set correctly like the solution of this thread, and after some searchs, it looks like this “Server Custom Event” must be set on the Player Controller or Character.
I don’t find the source of this info, but after moving the “Server Custom Event” from the Projectile actor Blueprint to the Character Blueprint, it worked!