Maybe raspian buster is the problem?

You can forget that, all my Raspberries (2, 3 & 4) run Buster. Have you tried the setting that is discussed here?

Next try:

I edited /lib/systemd/system/boinc-client.service to include the Environment parameter (as the error points to locales).

[Unit]
Description=Berkeley Open Infrastructure Network Computing Client
Documentation=man:boinc(1)
After=network-online.target

[Service]
*Added line to test whether inclusion of the Environment parameter helps
Environment=LC_ALL=C
ProtectHome=true
Type=simple
Nice=10
User=boinc
WorkingDirectory=/var/lib/boinc
ExecStart=/usr/bin/boinc
ExecStop=/usr/bin/boinccmd --quit
ExecReload=/usr/bin/boinccmd --read_cc_config
ExecStopPost=/bin/rm -f lockfile
IOSchedulingClass=idle