I think "XHTML For Digital Editions" is for ID CS3 only - probably it can't work with ID CS4 ...
you need to uninstall this software - XHTML For Digital Editions - or manually delete this catalog - but ONLY "XHTML For Digital Editions" catalog from "Scripts" catalog
or you can go to this file "OEBExportMenuItemLoader.jsx" and check line 428 ...
Try to temporarily remove OEBExportMenuItemLoader.jsx file from Startup Scripts folder and restart ID. See if the problem has gone.
Is it possible that your script preferences have been changed to use an earlier version of the scripting object model? If that's the case, then the following one-line script:
app.scriptPreferences.version = 6.0;
...might fix the problem.