I have a surface model of a truck I am trying to thicken, 0.1 inches inward, but it won't. I have been trying to thicken the cab part, which includes the outer body panels, and the underside of the cab with the transmission tunnel and another tunnel to the right (see first image). After rolling back the design tree and doing some diagnosing, I found a problem area on the drivers side of the tranny tunnel (circled in both pics). In the second pic, which is looking at the truck from the opposite direction as the first picture, there is an arrow pointing at the edge causing this issue. If I move this bottom edge forward from the corner about 0.2 inches, the thicken works. But if the edge is right at the corner, the thicken doesn't work. I get a rebuild error saying "Unable to thicken surface" or "Failed to offset face or couldn't be deleted."

To fix this, I've tried adding a fillet at this edge in the drawing for this cuttout. It will thicken with the edge right up to the begining of the fillet, but when I roll down to the next surface and try to thicken (after fixing this surface to include the fillet in the boundary edge), I get the same errors.

Are there any relatively simple ways to fix this? Or should I rebuild the transmission tunnel so it doesn't have any hard edges?