Hi Achille,
to address this open Local Data Manager from the Teamwork/Project menu and delete the local cache of the affected project. After that you should be able to join the project again.
Such can happen if the join process is somehow interrupted and the local cache is corrupt. We are working on this to help to avoid similar situations.
Note that by deleting the local cache any not sent data will be lost.
I hope this helps,
Marton
Marton Kiss
Chief Product Officer
GRAPHISOFT