Yeah, this is a tough one to see, but I think I spotted it.
If you look at this part along the red line that I added, and then imagine your cylinder sitting in there, that red edge just barely touches the cylinder, and has air on both the left and right sides. This creates the zero thickness condition. The answer to this is to build up a small amount of material on the left side of the red line that acts maybe as a fillet or a rib or a wall thickness. It's hard to say what you're using this for, but if it's a single piece of material, you're going to get something ugly at the red line where you have a feather edge. CAD doesn't like this kind of stuff. You could also solve this by making the new cylindrical extrusion as a separate body, or putting it together as an assembly. It really depends on how you're going to manufacture/document this.