A family of Microsoft relational database management systems designed for ease of use.
Costas,
The first question is, do you have SP2 installed for Project 2010 and SP3 installed for Project 2007? If you do not, all "bets" are off, you need to install those updates immediately as they are critical to cross-version compatibility.
Even if you do have all updates installed, working between versions, particularly Project 2010 and above with versions prior to Project 2010, is not the most ideal scenario. It's better to settle on a particular version and stick with that.
That being said, it is possible your file became corrupt. If you have all updates installed, try saving the Project 2010 file as XML and then re-open in Project 2010. That process usually clears corruption, but not always. Note: by using this process you will lose any custom formatting.
Hope this helps.
John