First, I must thank the team at Fog Creek for making the wiki much more usable. The editor is truly a pleasure now. It's so easy, our salesman is now using it to document customer requirements for custom integrations.
And now to my issue.
If I edit a mockup, save it, return to the wiki edit screen and then try to edit the mockup again, the mockup editor comes up with a blank screen. As long as I cancel and exit, the original mockup is left intact. That is, if I go into view mode, I see the mockup. I have not had the nerve to save the blank mockup to see if it replaces the one I created in the first edit session.
As far as I can tell, this problem only occurs if I try to edit the same mockup twice within a wiki page editing session. On a page with two mockups, I was able to edit the first, save it and then edit the second properly. However, going back into the first brings up the empty mockup editor again.