What is “Self Reference is not compatible with PLACEHOLDER-CLASS” ?
When I Play in “Editor PIE” everything is OK, but Play in “Standalone Game” will get this error.
Same problem here! Haven’t solved it yet.
I found a workaround if you are still interested: inside your ServerBuffStartCallback change you BuffActor type to just Actor. Then after you receive that argument cast it as needed class.
Delete Intermediate, DerivedDataCache and Saved directories, everything is OK!