If you are trying to open a project from within DVDA that's had the source files moved, you can get this message. Try opening it by double clicking it without DVDA being open. You should then get a 'can't find' message. Just navigate to the first source file. If all the others are there, should be job done. If not, you'll have to do them one at a time as DVDA asks for their location.
I did that... and when it opens I get a blank box where i can see it looked for something but it stays white ( nothing happens ). I hit the x botton to close that box after a few minutes and that is where the message I get the error not a valid project so I don't know if it was asking to find the files...
Is the .dar in the same folder as the source files? If not try that. Leave DVDA for a while to see if it sorts itself. I've waited 20mins or so before when I've changed something.