I have 9 ePubs which I opened with your editor and added the words "ADDED TEXT" to the description meta data. All of them had the same zipping error and the OPF error.
One, which had been run through Calibre, lost the date meta data.
The calibre meta data that was dropped was
<dc:date>2010-12-31T13:00:00+00:00</dc:date>
I can't post the ePubs in a public space but I'll email one to you.
Mark
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