Something is not wrong with slackpkg.... something is wrong with your computer.
Start with copy/pasting the exact output into this thread so that other people can try to think of a cause.
What you also must mention is the output of these commands:
ls -la /var/log/packages/slackpkg*
ls -la /var/log/packages/pkgtool*
ls -la /var/log/packages/grep*