There is something wrong going on here. Until I figure it out, you might be able to work around the issue by uninstalling PopSearch, deleting its local database, and then reinstalling PopSearch. The local database can be found in ~/Library/Safari/LocalStorage, with the filename:
safari-extension_com.canisbos.popsearch-zanvztser6_0.localstorage
Please let me know if that doesn't work.
Chul
Version 1.6.7 should work for you. You can get it from:
http://dl.dropbox.com/u/23990/Canisbos/PopSearch/backup/PopSearch167.safariextz
You'll need to uninstall PopSearch first, as Safari won't let you downgrade to a lower version.
I wish I could debug this problem you and the other user encountered, but I don't know how to duplicate it.
Chul