I saw also an other IE8 phenomenon on an other windows-computer comes an Meldung: 'null' ist Null oder kein Objekt Zeile: 1403
An unminified version of Openlayers.js would be more helpful to diagnose the error. As far as i understand the error is thrown when a variable that is not an Object is accessed like an object i.e. "a.property" with "a" being null for example. In the current case I can only speculate that either "this" or "feature" is null (or not an object)/ Daniel