ah... that makes more sense. Sorry for the confusion.
Is your report saved with data or saved without data?
If saved with data, it should just export as the report was saved.
If the report does not have saved data, then it requires a new instance to be generated. This is where I ran into an error similar to what you are seeing.
If generating an instance without providing any parameter info in the request body I get:
Some parameters are missing values (CRR 00107, JRC00000169)
and if I include the parameter name in the body without any value I get:
Parameter name 'myParam' given with no values. (CRR 00019)
I believe this is a defect in that the refresh (creating an instance) does not appear to account for whether or not the parameter is Optional (ie there is no attribute in the response indicating if the prompt is optional, so it cannot be set.)
Dan