Keithley 2420 and SR844 via Keysight GPIB cables to NI PCIe-GPIB card on Ubuntu 21.04

Hi,

I have been using a NI PCIe-GPIB card on Ubuntu 21.04 to control instruments. My Keithley 2460 is working fine. I can write, read and query. The SR844 times out frequently but I can write on it. The Keithley 2420 does not work at all. It times out and complains interrupted signal when sending it again. 

I am using Linux GPIB and pyvisa to control the instruments. They are connected via Keysight cables. 

Any suggestions how to get around this? Why is every instrument behaving differently? Are the Keysight GPIB cables incompatible?

Cheers,

Robert