Error 1073807346 Serial Modbus

Hi everybody,

I'm trying to acquire data from an Infineon XMC 4500 microcontroller with serial Modbus protocol (9600 8 N 1); I also use a USB-RS232 converter.

When I read registers I get a 1073807346 Error ("The given session or object reference is invalid"). In particular the error appears in Read Holding Registers and Read Input Registers functions.

I checked all possible solutions from NI suopport document relative to this kind of error.

Does anybody ever dealt with this problem? 

I also include a VI snippet from selection.

Thanks any advance for any advice.

Andrea.