Simon,
I've been using ywriter for years, mostly yWriter5. I have a book of poetry that I add to occasionally, so it's been around a while. I recently loaded yWriter7 in one of my VM's to access the poetry folder and post selections to a social media group, and it reported that it could not unpack the file. (It previously had opened it without error several times.)
I went back to the original computer and opened with yW5 and found that it had a corrupt scene (fortunately in an archive chapter, not the published version.)
Anyway, I deleted the offending scene and still can't unpack it in yW7.
Let me know if you want me to help you diagnose this.