I’m seeing the same thing during cook, after updating to 4.26.
Fortunately the error also occurred when loading my game map in editor so I was able to easily breakpoint the engine code and isolate the corrupt asset - in my case my HUD blueprint.
Specifically it was a UserWidget object reference property that was being parsed as a soft reference due to the fallback in FBlueprintEditorUtils::PropertyValueFromString_Direct().
Deleting the property and creating a new one fixed the problem for me.
If any UE4 devs pick this up, I would expect the property to be sanitised if the engine asserts, so the next serialisation writes back valid data for the property. It would also be VERY useful to have correct object context for the log.
Cheers!