You may have locked your design from further editing but in isolated cases, this could also encrypt the file's contents from being re-opened by DSM.
If you face this situation where the RSDOC just won't open, there is a way to unlock the design:
1. Using 7-Zip or another file archiver, extract the RSDOC. You will see the contents in a similar format:
2. Open the 'SpaceClaim' folder and then the 'document.xml' file in the Windows Notepad program.
3. Using the Find tool (CTRL + F), look for this text <monikerOriginalToReplacements_Values /><locked>True</locked>
4. Now, change the value from True to False : <monikerOriginalToReplacements_Values /><locked>False</locked>
5. Save the file (CTRL + S) and close it.
6. Go back a folder. Select all these contents and right-click on them. Click on Send to -> Compressed (zipped) folder
7. Finally, rename this new ZIP file along with the file extension to 'your preferred design name.rsdoc'
Check that this updated RSDOC opens in DSM.