Answer by Bobat

I figured it out. This issue isn't related to the version of Unity. The problem is that the Asus sync program was constantly re-loading its own version of adb.exe. As long as no other instances of adb.exe are running it works. The solution is to kill or uninstall the program (asusUPCTLoader.exe in this case) that keeps respawning adb.exe. I’ve seen many other people with other brands of devices complain about this same problem. Other devices will have their own custom programs that spawn adb.exe, fortunately it’s easy to find them. The best way to figure it out is to use ‘Process Explorer’, it's essentially a ‘super’ version task manager that lets you easily inspect all the processes, dlls and handles loaded in the system. This is an extremely valuable tool, no pro dev should be without it. Get it here: http://technet.microsoft.com/en-us/sysinternals/bb896653