Solved!!!
Installing an ADLINK USB-3488A with its Drivers parallel to an existing Driver (e.g. NI GPIB-USB-HS) ends up in an Situation, where no LabVIEW instance (VISA, GPIB etc.) can talk to the GPIB-Bus. In order to install a proper Version of each Driver the "old" or "unused" Drivers (NI and ADLINK) must be uninstalled, before installing the "new" Driver (NI or ADLINK).
Thank all for fast replies.
