Had to shut down machine today so took opportunity to upgrade to BOINC 7.0.60. On restarting would not connect to local host so I started from a terminal and got the message,

Unable to open gui_rpc_auth.cfg in the BOINC directory, which links to a file in /etc/.... I didn't copy the details and can't remember them. Having renamed this file by adding a 1 to the end of it, all worked normally. I am using the standalone version as opposed to the package manager version of BOINC.

I suspect there is a more complete and elegant way to describe what happened and the solution but it worked for me.