OK, I think I've found the culprit, or at least one of them. When I clear the conditional formatting from the first sheet, everything works (including the XSformatcleaner). When I add it back, it fails. Here's my CF code:

Code:
=NOT(IsFormula(B6))
The rule applies to a named range 'editable_values'. When a user overwrites the formula in a cell within this range with a value, the cells fill color is changed.