This error indicates that at some point after importing your project into TeamForge, you went into TeamForge and deleted the top-level task folder that was imported.
Once imported, meta data is added to your project file in MS Project that indicates which item ids are inside TeamForge that correlate to the items within your project plan. By deleting the top-level task folder, the synchronization method can no longer determine which items in TeamForge correspond to the items within your project plan. Hence, it cannot determine if there are pending changes. To correct this issue, open MS Project, access the Tools menu, and then select Unbind from the TeamForge options. This will remove the meta data from your project plan, effectively returning it to a pre-import state. You should now be able to re-import into TeamForge without error.