I checked the surface, and I can't find anything directly wrong with it, but judging from the results of the thicken and offset, I would say there has to be something squirrelly about it. Check says the minimum radius is off the part somewhere. I've seen that before, and it's just a bug in the tool, I think. To give you a better answer, I'd have to see how the surface was made. If you turn on the Curvature display, you can see little areas where the curvature doesn't change uniformly, although this might just be a display glitch or something.
If you really just need a thickened part and don't have any time for trying to find the reason for the error, then just loft a new surface. I'd make two planes parallel to the Top plane that section through the loop, then open a sketch and use Intersection Curve to capture the intersection between the plane and the surface, and then offset the resulting spline. If you do this twice, and then make guide curves in the other direction and use a Boundary surface, you should be able to rebuild the surface "accurately enough" depending on what you are using it for. Make sure that the new surface is taller than the original in both up and down directions. For the thickness faces, I'd use Ruled surfaces with the Normal To Surface option, and then use mutual trim to trim the boundary and the 2 Ruled surfaces.
Best of luck.
Thank you Matt, actually I redisigned all the thing again, but is strange how this tool shows one thing and then do another one, I design plastic products, and use a lot of surfaces to create soft and curvy stuff, but usually is a nightmare when I have to thicken them and I have to because I need to know the wheight of the parts. Thanks again.