Hide Textbox with nested IIF in RDLC expression

I'm new to reporting with rdlc.

I want to hide a Textbox if two statemt are true:

I already tried this expression:

= IIF(Not(String.IsNullOrEmpty(First(Fields!PartNumber.Value, "PackingData"))), IIF(First(Fields!IsDisposal.Value, "PackingData") = "False"))

But it doesn't work. I get this error message:

Microsoft.Reporting.WebForms.LocalProcessingException  An error occurred during local report processing.InnerException: The definition of the report '' is invalid.InnerException: An unexpected error occurred while compiling expressions. Native compiler return value: ‘[BC40000] \"RequestMinimum\" ist veraltet: \"Assembly level declarative security is obsolete and is no longer enforced by the CLR by default. See http://go.microsoft.com/fwlink/?LinkID=155570 for more information.\".’.

I hope someone can help me with this.