I compiled the latest master (8.1.0) and since upgrading get

"BOINC manager (pre-release)is not able to start a client. Please run the client and start again."

Starting the client with

sudo systemctl start boinc-client

either before or after starting the manager makes no difference. Tomorrow I will search git-hub for clues. Ubuntu 23.10 and 24.04 in a fresh install both exhibit this behaviour.

Just a quick look at the master source used for boinc-client.service.
There's this line:
ProtectHome=true

Didn't test it yet but you may try to use:
ProtectHome=false

Then run "sudo systemctl daemon-reload" and restart the client.

Tried that. No joy so far at least.

sudo systemctl status boinc-client gave

boinc-client.service - Berkeley Open Infrastructure Network Computing Client
     Loaded: loaded (/lib/systemd/system/boinc-client.service; disabled; preset: enabled)
     Active: failed (Result: exit-code) since Fri 2024-03-22 05:28:08 GMT; 29min ago
   Duration: 18ms
       Docs: man:boinc(1)
    Process: 160474 ExecStart=/usr/local/bin/boinc (code=exited, status=200/CHDIR)
    Process: 160475 ExecStopPost=/bin/rm -f lockfile (code=exited, status=200/CHDIR)
   Main PID: 160474 (code=exited, status=200/CHDIR)
        CPU: 34ms

Mar 22 05:28:08 Swarm systemd[1]: Started boinc-client.service - Berkeley Open Infrastructure Network Computing Client.
Mar 22 05:28:08 Swarm (boinc)[160474]: boinc-client.service: Changing to the requested working directory failed: No such file>
Mar 22 05:28:08 Swarm (boinc)[160474]: boinc-client.service: Failed at step CHDIR spawning /usr/local/bin/boinc: No such file>
Mar 22 05:28:08 Swarm systemd[1]: boinc-client.service: Main process exited, code=exited, status=200/CHDIR
Mar 22 05:28:08 Swarm (rm)[160475]: boinc-client.service: Changing to the requested working directory failed: No such file or>
Mar 22 05:28:08 Swarm (rm)[160475]: boinc-client.service: Failed at step CHDIR spawning /bin/rm: No such file or directory
Mar 22 05:28:08 Swarm systemd[1]: boinc-client.service: Control process exited, code=exited, status=200/CHDIR
Mar 22 05:28:08 Swarm systemd[1]: boinc-client.service: Failed with result 'exit-code'.
lines 1-18/18 (END)

Switching to packaged version for client (7.20.1) from memory, all works fine so issue is with client rather than manager which is still 8.0.1. I will do a new build with VM as that was with a minimal install which resulted in having to look for a lot more libraries before I could get the build to work. I will try again with a full clean install.

Update: 8.1.0 now running from usr directory in VM. Not best security practice according to some, I know but as the VM is only used for BOINC not an issue.

Yes. BOINC should add required packages as dependencies.