Why does the device give an error (catching data from the COM port)?

Question

C++ / C#

Hello! From one device I catch data via (USB). I'm still a beginner and don't understand much. Therefore, those who dealt with this issue and can help me, it would be great!
I connect the device to the laptop and install the driver. Install virtual com port driver. In the device manager in the properties of the com port, the flow rate is 9600, and so on.
Install com port toolkit. I configure in accordance with the settings of the virtual com.
The data is coming. ASCII (the field displays the same values ​​that the device transmits).
But at the end of the data transfer, the device gives an error "An error occurred while transferring data."
Question: what am I doing wrong?
Who can explain? Maybe at the end of the data transfer, what thread should be sent a signal to the device?