Persistent error messages when splitting compound wall into regions

[Tibor]

I'm trying to create a split region (to show as a soldier course) at the top of a brick cavity wall and I keep getting this cryptic error message, then the routine aborts. I get the same error message when trying to add a split region in the middle of the wall. However, several split regions towards the bottom of the wall went through OK.
Any ideas why the error message and how to make this work?

[Tibor]

I sort of solved the problem - recreated the wall from scratch and was able to make it work. But I would still like to know wnat the error message meant.