Weird grey surfaces appear in final render

I was rendering an animation and in some of the frames appear those grey areas between some of the vertexes.
Has anyone an idea what might be causing this?

Thank you in advance for any help.

Why no .blend file supplied with your post for review ? Deliberately made more difficult for anyone to be able to help you !

Is your model have faces with > 4 sides. If so these will get triangulated at render time and you may get results you may not have wished for.
Do you have any hidden vertices
What modifiers are you using. Have you set them to have different settings for the viewport and render
Do you have any hidden objects (check outliner)

QUADS. or use the triangulate modifier.