I ran into the same problem, just want to leave one of the possible solution.
In my case, I did not turn on the GPIB mode on the E3631, unlike other agilent instruments I used, this one is set as RS232 mode by default.
Press I/O config button, If “RS-232” appears, choose
“GPIB / 488” by turning the knob to the right.
Then you are good to go.
