This issue seems to have popped up in our confluence install as well...
Confluence 2.10.3, MySQL
Gliffy 1.4.1
One small difference though, the space name prints instead of {0}. Here's the error message:
Gliffy Version 1.4.1 Macro Error
You don't have permission to view this diagram from space PROD
Edit: I don't know if this will help, but it looks like the creator of the diagram may have tried to move it to another page.
Edit #2: It turns out that yes, the user's attempt to move the diagram caused this issue. The user tried to copy the wiki markup from the original wiki page where the diagram was created. The new page displayed the gliffy diagram properly for that user and for the administrators but everyone else got the permission error I mentioned above. I was able to fix this issue by properly moving the diagram through these steps: http://www.gliffy.com/blog/2008/01/10/moving-gliffy-diagrams/
Diagram doesn't show and this error does. As confluence admin diagram shows fine or if users create new diagram on same page. Looks like a links is broken in the database between image and permission groups. Is it possible to check the database and populate it with a value?