Rob
Ansys Employee

Assuming you have three non-overlapping faces in the geometry tool, and meshed in Ansys (Workbench) Meshing I suspect you've hit a tolerance hiccup in Meshing. In the contacts part of the tree delete the contacts. The hiccup is that the domain length is high relative to the thickness so it's trying to remove the thin part.  Fluent does have tolerance limits, but not at the dimension ranges you're looking at. 

You may also want to read up on thin walls and shell conduction in Fluent.