I guess you have downloaded the server files instead of client, therefore your texture files are missing (no servers need any textures or sounds to load and handle a map)
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I don't think this is the case, 1 screenshot clearly shows that level terrain and sky texture are missing.
The transparent armoured glass texture of the StuG that he has been driving is missing as well, so probably that's why neither ground nor sky is visible. Trees are transparent so they glitch through other textures that are supposed to be transparent.