Tried several times using the original file, but could just not reproduce the error situation. I had several chapter-5.1.1.1.1.1.1.1 files, but now they where smoothly batch renamed without any hitch.
I did quite a substantial edit session just before the indicated error occurred so that might also have contributed to the problem situation. Anyhow, it seems that it is unlikely to occur very often. When I encounter it again, I will certainly save the debug info now.
All: have a merry Christmas!
