Hi Michael,
thank you very much for your advice. I am aware that using SNMP v3 without any authentication is unsecure. We already had tried setting the option authNoPriv - without success. The SNMP access failed again. It only works without passphrases. Very dirty. 
As soon as I have time, I will test your recommendation again.
Unfortunately, HP didn’t document any passphrase policies for this feature. Or did you find any?
Thanx.
Cheers,
Antje