Creation Kit. Replacement of a 3d Weapon Model

[ProsecutorGeneral]

I want to replace a model of a crossbow . But when replcaing it I fail. Creation kit tells me
Masterfile(BSStream Error) NIMaterialProperty cannot find create function. in meshes / my directory / file.nif

Then the 3d View shows me a red square instead of a Model with an exclamation mark in it.

What is wrong.