I have tested it on my local environment using the similar expression on my report, we cannot reproduce this issue. And it seems that the syntax is OK. From the error message "The Hidden expression for the tablix 'Tablix3' contains an error: The query
returned no rows. The expression therefore evaluates to null", it seems that the first IIF statement cause this error. Please try to remove the first expression and check whether this issue is persists or not. Besides, please ensure that there are
some records when you use the parameter to filter the report data.
If the issue persists, please elaborate your report structure, if possible could you please post both the report and the dataset with sample data to us by the following E-mail address. Then, I can try to reproduce the issue again and for further analysis.
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