gui_rpc_auth.cfg is empty on the systems in my boinc farm as I don't use a password. I was unable to use boinccmd.exe to access them and I tried the following before giving up and putting in a password.

boinccmd --host 192.168.1.151 --quit
boinccmd --host 192.168.1.151 --passwd "" --quit
boinccmd --host 192.168.1.151 --passwd '' --quit

The first 2 above gave failure to read() error, the 3rd one said the password was wrong.

I also tried 192.168.1.151:31416

I went and put 1234 in that gui_rpc_auth file on that 151 host, rebooted, and the following worked
boinccmd --host 192.168.1.151 --passwd 1234 --quit

It seems to me that leaving off the -passwd should have worked but that is just my 2c