Florent:
In general, if it works from MAX (Open VISA Session) then it will work from LabVIEW. As the other post suggested, increasing the timeout usually helps, since the default is 2 seconds. Another thing to try is to right-click on the VISA Read node and change it from asynchronous to synchronous. Sometimes this helps.
If that still doesn't work, I suggest sending our support folks 2 log files from NI Spy: 1 from the Open VISA Session, which works; and 1 from LabVIEW, which doesn't. That might help our tech support see what is going on.
Dan Mondrik
Senior Software Engineer, NI-VISA
National Instruments