Sometimes a MagicDraw/Cameo .mdzip project file can be corrupted, such as by using non-compatible Element names:
If you do a accidentally break an MDZIP file using bad characters in the model, you can fix them relatively easily by using the unzip/edit/re-zip procedure described here, noting that the "BINARIES" are in fact just XML files that you can edit directly.
Look for the file com.nomagic.magicdraw.uml_model.model, edit it as an XML file to fix it, then re-ZIP, making sure you ZIP the entire collection of files (including the one edited one) as a selection of all files, NOT any parent folder you may have unzipped into. Then rename the new archive file's suffix from .zip to .mdzip and it will re-open in MagicDraw/Cameo (assuming your FIX edits were OK).