Well, you got me looking in the right direction, although the fix for me is still unknown. I started looking for anything that might use SlateCore, but didn’t find anything, but I did find a struct that the system was complaining about bad initialization. In totally unrelated code. Fixed it, and two other issues were fixed in the process. The slatecore error is gone, and the UMG MVVM plugin started working properly (really not sure, as the struct I fixed was in a time system plugin I am writting, that doesn’t even touch either of these other systems)
So thanks for helping, even if it wasn’t directly 
Maybe OP can get an idea here, or someone else in the future can find this useful.