I've installed BOINC, but I get an error message when starting the BOINC manager:
BOINC Manager (Pre-release) - Daemon Start Failed
BOINC Manager (Pre-release) is not able to start a BOINC client. Please run the client and try again.

How does one run the client?

BOINC Version: 7.18.1 (x64)
wxWidgets Version: 3.0.5

You can't - unfortunately, v7.18.1 was never intended for Linux release, but the Ubuntu guys picked it up and released it anyway. It's broken.

Two options:
a) I posted a workround in https://boinc.berkeley.edu/forum_thread.php?id=14792 yesterday.
b) Wait until the powers-that-be have actioned my request in https://github.com/BOINC/boinc/issues/4930. That may be slower.

sudo systemctl start boinc-client

Edit - What Richard said. Typing the above in a terminal will start a valid client however.

I've installed BOINC, but I get an error message when starting the BOINC manager:
BOINC Manager (Pre-release) - Daemon Start Failed
BOINC Manager (Pre-release) is not able to start a BOINC client. Please run the client and try again.

How does one run the client?

BOINC Version: 7.18.1 (x64)
wxWidgets Version: 3.0.5

What is the result of running "sudo systemctl status boinc-client"?
If it is disabled, you must first enable it: sudo systemctl enable boinc-client

When the package is installed, it will have created a user and user group, both named boinc. To be able to use the Boinc Manager, you must add your own user to group boinc. I don't know how to do that in Ubuntu.
If you have installed VirtualBox so you can run VMs inside Boinc, that package should have created a user group named something like vboxusers (that is what it is named in openSUSE). You must add user boinc to that group, or the client will not be allowed to run any VMs.

Finally, you should be ready to go: sudo systemctrl start boinc-client.

If things are set up in any way similar to what they are in openSUSE, that should be the last time you ever need to start the boinc client this way. Henceforth, it should be started automatically whenever the systemd target multi.user (aka runlevel 5) is reached.

Thanks, that did it. Can now connect to projects.