Please attach the DXF file, thanks.

Attached file is made toolpath one by one, without a generate all
An generate all should produce the error.

Double checking creating single toolpaths the same Inside / outside error also appeared.

Sometimes the selection gets lost, sometimes the toolpath-attributes were not updated.

There are two problems:
- The contour consists of loose entities which leads to different results, depending on where the algorithm starts identifying contours.
- There are tiny gaps in the contour. Usually not a problem, but in this case, it can lead to the algorithm not being able to determine the orientation of the contour.

Both problems can be solved by converting the contour to a polyline (preferably logically closed). Polylines have no gaps (by definition) and the start and end are clearly defined.

Sounds to be the cause, however,

I tested closed lining (layer outline) by trying a full hatch off all outline entities; if that's goes well then all entities are end-&-startpoint alligned; polylines should not be needed at all.

Tiny gaps in the contour should not been there; the original drawing is an Acad R10 dxf exported. Which entities (handles) are gapped? So I can check where in the proces these gaps appear.

The big surprise here is not a failing contour (by gaps), but the fact that creating a single toolpath goes OK, but that is does not when using Regenerating All. This implies a change in the toolpath's start- or endpoint and/or selection; and that is odd.

If there are arcs involved there are always tiny gaps. Arc calculations involve PI, an infinite fraction that cannot be represented accurately by computers. Polylines work around this limitation by eliminating the duplicate presence of vectors (i.e. defining vertices instead of segments).

This implies a change in the toolpath's start- or endpoint and/or selection; and that is odd.

Again: this has to do with the fact that the contour is defined by loose entities. No start point is defined and the algorithm gets to choose everytime you regenerate where to start. If you regenerate multiple times, you will get different results. This information is not stored in DXF either. The first generation can be correct or wrong but just happened to be correct in your case.