It's been a few weeks since Slackware's french mirror site mirror.ovh.net/mirrors/ftp.slackware.com keeps spewing out checksum errors with slackpkg. It's not any misconfiguration on my behalf, since it's sufficient to replace the OVH mirror by another one just below, for example (ftp://fu-berlin.de/unix/linux/slackware
==> deutsche Qualität, heh) and everything works perfectly. On a side note, OVH has a bad reputation for non-existent QoS here in France.
I suggest the slackpkg maintainer contacts OVH (and sees if he can be lucky to get any response from them).