It doesn't look overly complicated but it only takes one invalid where the boundary somehow loops over itself.
You can't always see the problem but if I had to guess I would be looking here (just a guess though).
You can try splitting the slab up into smaller portions to see where the problem may be.
Work on a copy so it doesn't matter if you destroy it while tracking down the problem.
Or sometimes it is just easy to delete and re-create the element.
Barry.
One of the forum moderators.
Versions 6.5 to 26
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