We are in the process of transitioning a Project Server from v2007 to v2010. Our Server is currently in v2010 with backward compatibility mode turned on while we transition all our users to 2010 clients.
We have a file that opens fine and saves fine in v2007 clients, however when we open it in a 2010 client it will not save. There is no indicator at the bottom of the window that it is saving and the Server queue never registers the save request. The file will also not check-in with a 2010 client. The problem so far seems to be isolated to a single file, so it is not systemic.