Is this all instapaper created ePub files, or just a particular one. I tried a single file from Instapaper and it opened up with no problem. Most likely either
a.) This is a problem with ePub generation that instapaper already fixed (they marked this a public beta)
b.) Or it was a particular article that was badly form XHTML that caused ADE problem (however I agree that ADE shouldn't crash).
Following your suggestion I tried an ePub with different articles and limited myself to those in English language. This ePub worked flawlessly.
You were right, that my initial attempt included some invalid markup. When I transferred the broken file to my PRS 505 using Calibre the Sony reader reset itself immediately when I selected it.
It would be interesting to know the article that caused the crash, or if you don't want to go to the trouble, then getting a copy of the file that caused the crashes. I'll send you a private message with my email address if you are interested in sending the file along.
The instapaper folks may be interested as well.
SInce I've since overwritten the crashing file I have to check my Time Machine backup.
Meanwhile I will try to create a separate ePub from every single article in question to make debugging easier. As soon as I find the article in question I will send it to you.
Thank you very much for your help!