Without the dll or hardware, I can only speculate. Since there is a function to open the USb port, see if there is a function to close it when the VI stops.
You should also make some changes to your VI. The mechanical action of the OK button should be changed to Latch when Released like the stop button is. Then you would not need those Value properties. Place a wait inside each of your while loops. You are just wasting cpu cycles by polling so often. Make the com port control an I32. Eliminate the sequence structure. Think about using an event structure instead pf polling the Booleans
