Cannot talk to klauncher

Could not start the process Cannot talk to klauncher: Did not receive a reply. Possible causes include : the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken

This has happend to me at least three times before, though not regularly. Today it happened on two successive attempts to boot. The third came up as usual.

Netrunner begins as usual, but stops with the panel, correctly set up, but before the intro sound, and with no running applications, though wallet does ask for its password. Firefox, manually started, remembers what it was doing last night OK.

Reboot, shutdown, and logout via the menu don’t work, though the first attempt did set up a flurry of disk accesses. The various solutions I found online, such as using Ctrl+Alt+Backspace, Alt+F2, or dbus-launch klauncher do nothing at all - in the latter case klauncher could not be found.

# shutdown -r now does work.

Is there anything I can do to stop this happening?