OmniGraffle Pro 5.0 document cannot be saved
 

must have been a corrupt doc, as new docs save ok, as do recent revisions of the problem doc...
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Saving my doc first returns this message:
"The location of the document “homepages-04.graffle” cannot be determined. You can specify where to save it.
[cancel] [save as...]"

Choosing to save as returns this:
"The document “homepages-04.graffle” could not be saved as “homepages-04 [ok]”

This happens whether I change file names or not, and whether the doc is saved locally or to a networked drive... WTF?