Problem with update to Keil uVision 5 with STM32F4 discovery
After spending the hole afternoon, I finally managed to solve the issue. 
In my case, I was using the STM32F334 Discovery kit.

There is a Solder Pad that must be closed in order to be able to trace variables. (SB17 in STM32F334 Discovery kit). 

by default, this is open. 
I just soldered the pad, and it is working fine now.