Hey man I think I may have found a solution.
I seem to be in the exact boat as you on this one, but what I did was
- delete all nav mesh
- save project
- go and copy all the project files from where ever they are saved on your PC
- make a new project
- paste the project files from your last project into your new project
- apply nav mesh again (and dont forget to press P!)
I hope it works for you like it did for me!