I keep getting this error when I try to launch a test session. The object itself was dragged and drop into my world form the content drawer in UEFN. This has now happened with many different objects that are take right form the default content library in UEFN. There dose seam to be a work around if you first place the object in question form within fortnite creative (game client). Dose anyone know if this is a bug of some kind or just UEFN acting odd? Any help would be great thx! 
delete the object- then save
then try to see if it doesn’t give you a load up error.
if it doesn’t head back to UEFN and place the object back in
Learning from experience you HAVE to avoid using cut,copy ,duplicate and paste as this seems to be giving this error. This Software is still unstable AF and i hope its still in Beta.
i’ve just had to manually delete over 145 crowd elements from my project due to the same error- i deleted every single one then it worked.
i now have a different error -something to do with Fortniter servers… ugh