ClassiCube can load lvl files, but the lvl file doesn't contain additional block definitions (so "everything else" goes green). Those exist in a blockdefs.json or lvl_name.json file (it doesn't look like CC can load these extras).
The cause of the "end of stream" message tends to be a bit vague because of the compression and there's not much that can be done about that.
However, there are other tools that can extract whatever is left in the file starting with things like using "gzip" to test if the file has been truncated or otherwise corrupted. Depending on the nature of the corruption it may be possible to extract the entire map (... or nothing at all) with other tools.
Note: The spawn position is right at the start of the file and you got that.