"... just close dude and open it again, ..."
In my case restarting client didn't help at all.
I was testing a bit in the meantime and I think that The Dude has major problem with working with SNMP v3. Tested both with and without encryption.
I guess, even lenght of the encryption key and password may matter because I have experienced some odd behaviour depending on lenghts of the keys/passwords.
I didn't test long enough to find pattern, but maybe I will some day.