We’re aware of this issue and are working on a fix. Are you working primarily in Creative Mode rather than in UEFN editor? That might explain the validation errors.
For anyone else that sees this thread with Validation errors related to Physics Materials:
The issue stems from placing assets in Live Edit rather than UEFN. If you place an asset with foliage or some cloth related asset in Live Edit and then exit Live Edit you might hit this issue.
My repro steps were:
Open Project
Launch Live-Edit
Add any prefab that has an asset that references a Physics Material
Logout of Live Edit
(Close Fortnite client)
Try to launch session in UEFN again
Hit validatio…