The user.config file is corrupt. This can happen if your computer crashed or something while RD Tabs was saving the config.You can try hand-editing the user.config file in the appdata folder for RDTabs if you have data you need to save, or you can run the 32-bit version, go to Tools -> Profile Cleanup, and then delete the newest versions. When you launch RD Tabs (64-bit) again it will revert to the previous version.