We were able to reproduce the problem and a fix has been created.
The problem is occurring because the SaveAs method uses optional parameters, as do a number of these Interop methods and if not actually supplied to the method the compiler was generating a null parameter instead of the expected value of System.Reflection.Missing::Value.
Please create a support incident and place my name in the description and I will ensure that it gets registered against the rpi so that I can get a fix to you when available.
In the meantime you can workaround the issue by specifying the parameters as follows, although you might have to do this for other Excel Interop methods as well.
invoke ExcelWorkBook::SaveAs("\temp\test1.xlsx"
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type XlSaveAsAccessMode::xlNoChange
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
type System.Reflection.Missing::Value
)
Thanks