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 validation errors for various Physics Materials (so far I’ve seen foliage and cloth)
If you have a workflow like this please consider using UEFN to place foliage or avoid placing foliage/cloth in Fortnite (rather than UEFN).
Foliage and Cloth materials are the assets we’re currently aware of. If anyone sees any others please let me know. In order to get around the validation errors please use the method I mention above of searching the outliner for the uasset name to find the actors you need to delete.
We’re aware of this issue and a fix is already in the works. Sorry for the hassle and props to @Avenom for the repro steps, you get a huge gold star 
EDIT: This issue appears to apply to many different assets and situations. Chests/Ammo boxes that are copied in Live-Edit might spawn a rare version which can throw Validation errors (fix already on the way). Destroying a Produce box from the Chest and Ammo Gallery can spawn a FortPickupAthena actor which might be un-delete-able and cause validation issues @MichaelRicks. See below for other examples but for now I’d be careful when editing in Live-Edit. We’re working diligently to get these fixed for you.