Edit: solved it by uncompressing locally and recompressing with winrar.
I use control panel to unzip/untar (RC5) and in both cases I get errors:
/bin/gtar: This does not look like a tar archive
/bin/gtar: Skipping to next header
/bin/gtar: Archive contains obsolescent base-64 headers
/bin/gtar: Error exit delayed from previous errors
Archive: forum/smf_2-0-rc5_upgrade.zip
End-of-central-directory signature not found. Either this file is not
a zipfile, or it constitutes one disk of a multi-part archive. In the
latter case the central directory and zipfile comment will be found on
the last disk(s) of this archive.
note: forum/smf_2-0-rc5_upgrade.zip may be a plain executable, not an archive
unzip: cannot find zipfile directory in one of forum/smf_2-0-rc5_upgrade.zip or
forum/smf_2-0-rc5_upgrade.zip.zip, and cannot find forum/smf_2-0-rc5_upgrade.zip.ZIP, period.