Go into App Manager and disable all the repositories. Exit App Manager, then re-launch it and enable the repositories again. That should force a redownload of all the repository data and fix the corrupted files.
For the future consider installing Faster Application Manager:http://my-maemo.com/software/applications.php?fldA
Instructions how to install applications via X-Terminal here:http://thenokiablog.com/2010/01/28/how-to-install-
Symbian specialist until 31/01/2013 with interests in Maemo & MeeGo but an aversion to Windows!