Hi pulp_user,
It sounds like your default level has become corrupt or perhaps has moved to a different directory than it was originally created in. I suggest making a back up of your Project Folder then deleting the Intermediate, Config and Saved folders.
Reopen the Project and, in your Project Settings, set the default map back to the original map you were working on. If it continues to crash, open the DefaultEngine.inil, and set the default map to none. Then go back and try and reopen that level after you open your project to a blank map.
Let me know if this does not resolve your issue.
.