I closed (or thought I had closed) the BOINC manager, without ending the tasks. When I opened the manager again to administer the tasks, I was presented with a dialog saying "Another instance of BOINC manager is already running on this computer. Please select a client to monitor", and asked for a host name and password.
What do I enter to make it connect to the client that is (presumably) still running?
This was caused because the boincmgr process was not ended when the manager was closed, as evidenced by ps -e | grep boinc showing the manager (and the boinc process) still running though the window was closed.
Assuming you didn't set it up otherwise, the host name to connect to the existing one is localhost and the password is blank.