Hi Kevin,
I can't recall what the error was precisely, except it was a file not found. I need to find time to investigate. Pretty sure it's either the way I've hooked PE into calibre, or its a calibre issue because I could PE the same book from Sigil's open with on the opf in the same book.
I've been roped into supervising my neighbours kids learning from home, whilst their mother copes with working from home - so I'm a bit short of time.
Damn it: I took a peek, calibre editor isn't unpacking the epub to the same temp folder where it puts the opf that goes to PE via its Open With. It seems to create a separate temp folder for every file that's opened via it's Open With. I can't even find the unpacked book
Maybe using PE from calibre isn't as easy as I thought, but why did I think it worked before - for me its not important, I use both editors.
Aside : I hope nobody thinks your checkpoint feature is synonymous with the calibre editor's checkpoint feature.
Take care - BR