Compile to eBook hangs

PDF and DOCX work fine, but compile to MOBI or ePub2 doesn’t start.

Here I’m about to click Export.

I get a progress bar at bottom left, but no progress.

Recent changes were
a) I switched to using Front Matter selected according to Compile Group
b) in eBook Front Matter, I used $include to point to a subset of the Print Front Matter documents.

Scrivener seems to use some CPU time, and there may be glacial progress (not sure), but at this rate it will take hours to complete an eBook compile.

The problem goes away when I duplicate documents rather than using $include.

Could you please zip up and attach (or send to AT a project that shows the issue?

Thanks and all the best,