There’s a problem with the underlying preferences system that tries to store the column settings (such as width and order). You can fix this by deleting those preferences and then resetting them. Copy the following into the Terminal in order to delete the preference settings:
defaults delete com.fcs.sitetagger "NSTableView Columns com.fcs.SiteTagger.maintableview"
You need to be logged into Furl’s online service. It’s best to be logged in via Safari. Once logged in, SiteTagger should be able to sync.