I'm wondering if you've got a typo, or more than one line uncommented, in /etc/slackpkg/mirrors, or grep is broken on that box. This isn't just a wild guess: the first sed command in /usr/sbin/slackpkg pipes your mirrors file into grep.
To find out exactly where this error is coming from, you can execute slackpkg like this (kill it as soon as you've spotted the error, which may be as early as the seventh line)
Code:
sh -x /usr/sbin/slackpkg update