Same kind of problem here, in my Old MacPro :
- cannot upload in any board ( Arduino Uno and Arduino micro), in /dev/cu...
- when I try to use the Serial Monitor, bluetooth connections are done ( my keyboard and my magic mouse were disconnected)
- in bash, just try a ' cat /dev/tty....', same BT crash
My old and dusty usb was OK
Something between Serial driver and ElCapitan BT ?