Is there anything in the logs?
I know that this message isn't very helpful - some things I've seen that it could be due to include:
* the project file isn't correct (i.e. it's 0KB, or it's been edited manually)
* the project file has a higher version than the database you're using (i.e. you're trying to import a project from a newer version into an older one).
Hopefully the logs will provide more information.
You can also try to import each project individually to find out which one is causing the trouble.