Asousa:
The serial ports have a question mark if VISA is unable to open them. Since only 1 process can access a given serial port at a given time, the OS is returning an in-use error, which NI-VISA returns as VI_ERROR_RSRC_BUSY (0xBFFF0072). You can verify whether this is the specific error by using NI Spy. I do not know what is causing this error, but it is likely because another process (perhaps your LabVIEW application) opened the serial port but did not close it.
Dan Mondrik
Senior Software Engineer, NI-VISA
National Instruments