I wonder if the problem is similar to the one with Wallace and Grommit described by "plasticman"
***here***, where there was a problem with folder names with Greek characters.
Of course it could also just be a failure of the copy "protection" to recognize a valid DVD considering the message you're getting.
The only English forum I found for the game is
***here***.
There is a thread in that forum about problems loading saves
***here***