It may be a corrupt report dictionary. I would rebuild the dictionary using these steps:
Tools->Customize->Customization Maintenance. Use this screen to export all of your modified reports to a package file. Then exit GP and rename the REPORTS.DIC file. If you use a shared REPORTS.DIC make sure everyone is out of GP. Then Tools->Customize->Customization Maintenance and import the package file.
Let us know how it goes.