Until epic fixes this, you can do this for now:
In your DefaultEngine.ini, turn off fully async nav data gathering:
[/Script/NavigationSystem.RecastNavMesh]
;UE5.5 crashes due to async.
bDoFullyAsyncNavDataGathering=False
(I have one person who told me that turning this option off via the editor settings UI didn’t work. It wasn’t until they applied it directly to the ini that it worked)