> Yes.  Or simply describe in your own words what you did after starting
> Emacs.

Sorry for the delay. Attached you find two short latex files for which
the problem occurs. I prefer to describe the steps.

    -  emacs -Q

    -  set-variable debug-on-error t

    -  open old.tex via find-file

    -  open new.tex via find-file

    -  in new tex run ediff-regions-wordswise

    -  enter buffer new.tex and buffer old.tex  

    -  the error described occurs.

Uwe Brauer