Just got this error message again.
The client in question is v7.16.11, public release, for Windows 7/x64.
The Manager which displayed the dialog is monitoring it remotely, using the artifact from PR #4041, installed 15 days ago and running continuously since then (mostly minimised to system tray).
The 'client RPC password' is in a gui_rpc_auth.cfg file datestamped 20 October 2006. Yup, I checked that very carefully - it'll be 14 years old next Tuesday (the data folder was retrieved from a failed machine, about 5 years ago).
So, whatever glitched, it wasn't the password. The machine was last rebooted on 9 October, to install a replacement KVM using a different video connection.
I thought we'd sorted this out in the runup to v7.16.11. Why is it still possible for BOINC to tell a blatant lie?