What is the version and patch level of your QC product? Find the patch level (not the build number) by logging into QC and going to Help > About, and then clicking on the Additional Info button.
There are several possible causes for this. In some cases it has been resolved by installing the latest product patch. In other cases it has been a matter of having the correct permissions to the location where the mapping information is stored on the local system - &#65279;&#65279;C:\ProgramFiles\&#6527 9;&#65279;Microsoft Office\Office12\XLSTART\TDAddin\Tasks.
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)