If the error is not due to an extremely steep layering, the reason is probably an overlap of elements in the superelement mesh. Indeed it is quite difficult to detect the location and fix the problem in the mesh. If you haven't refined the mesh after generation, the element numers are probably relatively ordered, so you might have a chance to find the location using the mesh inspector in 'mesh geometry'. However, it is still difficult to fix. I currently see only two possibilities without having to start from scratch: 1. To export the mesh, fix it in GIS and import the mesh again. However, all parameters have to exported and imported, too. 2. To try to fix the error in the ASCII fem file. Be careful with the latter option as you can mess up your model totally!