[Daaf]

I went through all the files in the ePub a second time, searching for ../Text/EndNotes.xhtml.

I manually copied each ../Text/EndNotes.xhtml line that I found into a spreadsheet. I wanted to compare them to one another easily, and to total the number of such lines that I found in each file.

Before exiting each file, I cross-checked the number of lines that I had found against the total produced by Sigil's Count All button (found at the bottom right of the Search function).

In the last file the Count All tally (29) differed from the number of lines (28) that I had copied.

So I searched only for EndNotes, and found the following line:

..Text/EndNotes.xhtml#Endnote41

Once I had changed this line to read

../Text/EndNotes.xhtml#Endnote41

the whole ePub validated correctly.

Thanks Toxaris, theducks, DiapDealer, and Jellby. Without your input and encouragement, I would still be stuck at the first validation error.