COM (serial-to-usb) device appears twice on Win7 device manager - only one instance working

Hi,

setup:
client - win7, vh 3.2.4
server - rpi2, vh 2.6.0, Prolific USB-to-serial device

Usually all works well, a single COM port appears on device manager on Win7.

Once in 2-3 days or so, I reach a case where the "COM" stops functioning.
Then, if selecting 'stop using' and the 'Auto-use port' on VH client, a second functional COM port is added to device manager.
(i.e. the 1'st 'non functioning' com port remains visible on the device manager list)

Further notes:
- The VH client continues to show only a single serial device connected.
- the "new"/working device has a different COM number (i.e. usually prior number +1).
- attempting to "disable" the 'non functional' port will result in freezing the device manager)
- b.t.w we are running many (>10) systems with the same setup. The issue reproduced on more than one specific setup.

Would appreciate your advice as how to approach debugging this.

Thank you.