I believe that this might relate to the character-encoding settings of the operating system. Basically the operating system has a default character-encoding setting, and Simulink reads this setting and uses it when creating new model files. However, this setting changes with different operating systems (e.g. Windows XP vs. Windows 7). If you want to use a different character-encoding than the operating system default, you should use the "slCharacterEncoding" command at the MATLAB prompt before opening any models.
If you try to open a model with a different character encoding, the error message should indicate what you should be using instead.
