Coop Fatal Error->CTD

After playing about 10-20 minutes in coop the guest crashes to the desktop with the following error message:
“Fatal error:
[File:C:\UE4\CSAND\Engine\Source\Runtime\CoreUObject\Private\Serialization\AsyncLoading.cpp] [Line:6604]
LoadPackageAsync failed to begin to load a package because the supplied package name was neither a valid long package name nor a filename of a map within a content folder:” "

This has happened to both me and my friend respectively when joining the others game.

This happens to us as well on the official launch version:

Version: #95365/17742