It is probably due to a corruption in the shellstyle somehow. I'm guessing you modified it to remove the shadow in the folderband? It can be as simple as a missing close tag.
If not, then perhaps one of the properties in that part (Explorer & Shell > Explorer > CommandModule > <1>) has been modified. Compare it with the original Aero theme in that part and compare the properties. If they match, then export the "Explorer & Shell > Explorer > CommandModule > <1>" image from your theme and import it into Aero, and test it there. Try using Aero's original shellstyle also. If you read my tip I explained how you can use your own shellstyle.dll, but that trick unfortunately is universal (or at least I think) in that every theme (.msstyles) you open in VSB will use that single shellstyle. So if you used my tip/trick, you will have to revert back to the original shellstyle.dll.
I can't pinpoint on the problem, I can try and help you by deductive reasoning.
Hope trying the above brings something conclusive