Problem solved (without a reboot).
It seems the rendition of HistoryCleaner.xdat distributed with v.1.8.0.451 is new (the first since last July). It is no longer backward compatible. The first rendition of the file worked with PT2007 and all versions of PT2008 up to this one. The new rendition no longer works with those versions.
Possibly the v.1.8.0.451 "silent" update wasn't necessary ...