Hello,
I’m having the same error after migrating a map I did on another project, I fixed blueprints and other things that were not compatible with the project, but I have this error left that prevent me to start the session:
" LogValkyrieValidation: Error: [16] Found disallowed object type /Script/NavigationSystem.NavigationSystemV1 "
I tried few steps to search for the error, make sure it wasn’t something on the map or on the project but it isn’t something I can delete, so I am stuck with that error 
If you know a fix, i’d be very happy 