LaTeX Error: File `foo.ent' not found

The .ent file seems to be created only if you actually have an endnote (that is, at least one); otherwise you get the error you describe, occurring at the point that the .ent file is to be read in with all the endnotes in it.

Best would be if the package either generated an empty file, when there are no endnotes, or checked for the file before trying to read it.

A workaround is to stick in a dummy "just in case" endnote, thus making sure there's at least one.