This has happened to me on two different occasions, slightly different steps but same outcome. In both cases, the end result is I have a diagram I cannot update. I can assure you there are no page permissions issues that are causing it as I am the admin for both.
Step 1 (both scenarios): Created a diagram but forgot to change the name before I started working
Step 2 (both scenarios): Completed the diagram and couldn't find a "Save As" option so saved it with the generic name "Untitled Diagram"
Step 3.1 (scenario #1): Decided I wanted the diagram on a different page than the one it was created on, so I removed the diagram from the original page and added to the new page. I could not edit the diagram from the new page, so I added it back to the original page and still no luck.
Step 3.2 (scenario #2): Tried to change the name of the diagram from the attachments page. After doing that I could not update the diagram... so I changed it back, still no luck.
I need to make changes to both and 'd really prefer to not start from scratch again, any help would be most appreciated.