Please do the following:
-save a PLN from your teamwork project for backup (Especially if you have unsent changes)
-quit ArchiCAD
-locate the following folder: Documents and Settings\USERNAME\Graphisoft\TW Data (This is the folder, where ArchiCAD keeps a local cache of the projects for fast and possibly offline operations)
-you will find a cache folder there, its name starts with your project's name
-delete this folder
The next time you start ArchiCAD and try to open the same project it won't find the cache information locally and will have to reload the project from the Server
If it fixes you problem, then you can try to migrate the changes (if any) from the saved PLN to the TW Project, but your only tool is Copy/Paste now.
Let me know if this helps.