Hi Gerhard,
before doing evaperf fn, authenticate with evaperf fnh.
Example:
evaperf fnh localhost my_username
"localhost" assumes you're doing this from the SAN management server, otherwise specify the server name. The account needs to be a member of the HP Storage Users or HP Storage Admins group on the SAN management server.
You'll be prompted for the password and then you can try your evaperf fn command.