Hi,
I developed this driver with E4401B and I do not have any troubles. Please can you use NI Spy for VISA and attach spy report.
NI Spy you can find in Start menu National Instruments>>Ni Spy. Press blue arrow icon. Now you can start Getting started example. In spy window you should see all VISA calls. When getting started example finish, stop capture in NI Spy and save report file.
Thanks,
Zdenek