Thanks for notification.
This is known bug issue in many pligg versions.
I will try to find solution to this by myself, but this is not important as you may think,
after all network is working fine with great results in all aspects without this.
About footprints, footprint is like unique thing which can determine and connect things in certain way.
For example online footprint can be adsense pub-id, or blogroll link, or footer link, or some other unique signature.
This pligg bug(no this is not an error) isn't footprint and I will explain you why it isn't.
When you click on the tag, "search.php" script is searching the database for results, using that tag as a search query.
This is same if you would type "something" and click on search.
So in our case script can't find any matches/results and search.php will return empty page,
that doesn't mean that that empty page is a footprint, at least computers/boots/spiders can't recognize that as a footprint,
as a matter of fact no one can say empty search page is a footprint, it is just empty search page for certain terms.