Hi,
Accroding to the error might be some non-numeric values are coming this date object values.Might be the reason it is not working when Alpha numeric values are coming in the prompt.
Create one webi report and drag only date object without any prompt and see how values are coming in the report.Check if any alpha numeric values are coming or not?
other part can you try without convert function in the universe object.
toDate(@Select(Test\Date obj))
Amit