thanks for the advice. That was the first thing I did, set the "Split file every" to 200000k which then resets to 300 ... which is in many cases not enough.
Even then I end up with 2 or more section files, having the split somewhere in the middle of a paragraph in Sigil. (For several reasons I cannot use the "Split before hx" options...)
I know of the 300k limitation. But as I've said, I'd like to do the file split later manually in Sigil.
Well, it seems I'll have to live with that.