BOINC Manager attempts to run only one copy (per user!) of itself.
It stores a lock file in your home directory, named "BOINC Manager-<username>".
It stores the pid of itself in there.
Unfortunately, it does not use the pid to check that it is still running.
I had some sort of crash that failed to remove the file.
On attempting to run boincmgr --verbose, it quietly goes away, with no message.
I made do with boinctui until I found out what was happening.

Solution:

rm BOINC\ Manager-*

Notes: The manager really should do this properly. At least check that the pid in the file is still running. The file should go in /var/lock properly speaking, as that gets cleared ... at least on reboot, these days. Also, I'm not entirely sure why it needs a lock file to start with?

Have to ask the developers.
Possibly the need is for when you start multiple BOINC clients.