If i start UE4 editor with one project and then open another instance of UE4 with another project, it gets confused.
To recreate
Open two copies of UE4 with different projects
Open the level blueprint on the first project
Try to open the level blueprint on the second project
UE4 thinks its already open and will not open the level blueprint from the second instance of UE4.
Close the first instance level blueprint
Try and open the level blueprint on the second instance
Again it does not open
Close the first instance of UE4
Try and open the level blueprint on the second instance
Again it does not open
Close all UE4 and Reopen just one copy and its fine.