Installed on ubuntu 16.04, nothing happens. Right after installing it might have been working 'better' but when i closed it it stayed in the task switcher alt-tab and wouldn't stay in the background, seems unacceptable to me so i killed it for a while. After re-start: BOINC Manager says "Unable to connect to the core client". Everything but Help and Notices buttons are greyed out. Clicking Notices opens empty window. Help goes to "https://boinc.berkeley.edu/wiki/Simple_view"

BOINC consists of two parts:
the Client, which runs in the background and does the work,
and the Manager, which is a gui that allows you to see what the client is doing.

These 2 parts "talk" to each other on port 31416, (Pi), and if they can't, then the Manager gives you that message.
This is often caused by anti malware programs. (And in Windows by some part of Windows grabbing that port first.)
Or it could just be permissions.

You get better options in the Advanced view.

Finally, there hasn't been any Linux models for a couple of years. If you want to run the ones that are available, just load up Wine, and run a Windows version of BOINC.
This works really well.

Another possibility:
When you killed it, you killed the client. Then when you started it again, only the Manager started.
In which case, there WON'T be anything for the gui to talk to.

When you killed it, you killed the client. Then when you started it again, only the Manager started.
In which case, there WON'T be anything for the gui to talk to.

I removed boinc-client from the list of programs that automagically start on my Ubuntu18.04 laptop because it stopped the windows one starting under Wine.

boinc client then didn't start when I started the manager.

Running the following command before or after starting the Linux Boinc Manager enabled all to work normally.

sudo /etc/init.d/boinc-client start^C

However as no work under Linux from CPDN. I am just running the windows version at present.