HFS runs on a file server, so I never see exactly when it pops up since I only look at the server once per day.
But I think there have been user uploads each time it happens.
Even if the upload was successful it seems the next time I look at the file server, the message is there.
Could the HFS installation be corrupt? I attempted to upload a fairly large file a few weeks ago and it stopped half way through. I just restarted the machine and never thought about it again.