Can the value of @ExceptionReportPath be set to C:\Temp\ ? It's not clear why it needs to have the value of C:\Reports\ if you're wanting to change it to C:\Temp\ at runtime. Not sure if that would make a difference with the issue at hand, though, because if there was an issue with how you defined C:\Temp\ in the expression, I'd expect so see some kind of syntax error.

You might double check that you've set both variables' 'Evaluate as Expression' property to True. I really doubt that's the issue but sometimes a really puzzling problem ends up being something small like that.

In the end, you may be best to go with Phil's suggestion.