UPDATE :
I tried a couple things and nothing worked. So I created a snapshot of my project using Unreal Revision Control. As a last resort, I then deleted the Saved and Intermediate folders located in “AppData > Local > UnrealEditorFortnite” after closing UEFN.
When I reopened UEFN, all my Editor settings were reset including my Project Folders location (from my F:// drive to my C:// drive) which showed me a “Sync” button under my project that I just backed up to Unreal Revision Control. Clicking Sync downloaded my project to my Drive again from my latest Unreal Revision Control snapshot. Now I had two folders (MyProject and a duplicate MyProject_).
After syncing, I tried to launch a session from the duplicate MyProject_. I got some Validation errors which I never got with my original project when launching and it pointed me to the device that was causing an issue in the Message Log. It had something to do with a popup dialog device still using a deleted widget reference that I had missed earlier. So I deleted that device from my original project and it launched without cook errors.
Important Note: Deleting the Saved and Intermediate folders can result in the loss of unsaved data. It’s advisable to back up everything before proceeding. This could affect your other projects as well (make sure all your projects are backed up by Unreal Revision Control as well just in case). This approach worked for me, but it might not be suitable for everyone.
Mine was a an “assetvalidationerror” and it might be different for you.