Okay, I found another bug in the importer.
In smf2_importer.xml, find:
$smf_attachments_dir = rtrim($smf_attachments_dir, '/');
and replace it with:
$attachment_UploadDir = @unserialize($smf_attachments_dir);
if (!empty($attachment_UploadDir[1]) and is_array($attachment_UploadDir))
$smf_attachments_dir = $attachment_UploadDir[1];