I've a vague memory of that kind of error being caused by a bad mesh -- those red elements might indicate an element connectivity problem. Possibly, higher displacements (due to changing the force units) brought out the mesh weaknesses. You could try lowering the forces until it does solve, then check max displacements to see if they are 'small'. Also try some mesh controls (e.g., larger min angles) for a different and more robust mesh.
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