Stefan,
What version of MAX are you using, and what version of GPIB driver are you using? If you are using MAX 1.1, you will need to update it with the MAX patch that is available on the link below:
http://digital.ni.com/softlib.nsf/16a19f446db24ce5862568d2005b971c/1d01cd39d8fbbd8386256974006a32e6?OpenDocument
Nasir