Arduino IDE v1.6.8 - Error opening serial port

Hi, I just install Arduino IDE v1.6.8 on my Win10 laptop. Trying to use it with Genuino UNO, downloaded few times of code, seem working fine.

But when I try to open serial monitor, sometime the serial monitor will fail to open and the status section of Arduino IDE shows:

Error opening serial port 'COM5'. (Port busy)

COM number is correct as I did not change the COM number since I have load sketch to Genuino UNO a few time and actually serial monitor works for few times.

BTW, I only see this with Arduino IDE v1.6.8. I have been using Arduino IDE v.1.6.7, no such problem. Anyone have similar problem?