Hi @lumluk!
Thanks for posting, and thank you for posting the full error trace. This is a problem with your system, not with the classifier itself. The key line is at the very bottom:
OSError: [Errno 28] No space left on device
So you are running out of disc space! This makes sense — the SILVA database is a good bit larger than greengenes, takes at least twice as much memory to run, and some of the temp outputs could also take up more disc space. So you are probably filling up your temp directory.
See here for details on diagnosing and fixing (try changing your temp dir most likely):