Issue:
The Network Management Card 3 (NMC 3) does not establish communications if a local   Command Line Interface (CLI) session is active while the card is rebooting. When the card has completed its reboot process, the following message is displayed:
Unable to open connection to COMx, Unable to open serial port.

Product:
Network Management Card 3 (AP9640, AP9641)

Environment: 
Windows operating systems connected to NMC via the USB serial console cable and a terminal program 

Solution:

Use Tera Term 4.100 as your terminal program. Alternatively, unplug and re-plug the USB serial console cable to regain communications.