I could run the service “boinc-client” (which was derived from a corresponding
software package) for a while as usual.
I noticed then that I could not control it by the application “BOINC Manager”
any more.

See also:
BOINC Manager interface shouldn't fully block user interaction when in
communicating with client dialog
https://github.com/BOINC/boinc/issues/4048#issuecomment-1712562997

I noticed then that the program “boinccmd” did also not present expected data
processing results.
Thus I extended exception handling and debug output in original source files so
that my understanding of the involved control flow could grow.
I determined that it was tried to call the function “XOpenDisplay(":0")”.

See also:
https://build.opensuse.org/package/show/openSUSE%3AFactory/boinc-client#comment-1829112
https://github.com/BOINC/boinc/blob/44295c1a50d84e44992d97b9e949700d3b9af2d1/client/hostinfo_unix.cpp#L2012

Test result example:
Sonne:~ # systemctl start boinc-client-variant.service && sleep 2 && systemctl
stop boinc-client-variant.service
Sonne:~ # XX=/tmp/boinc_log-one_day.txt &&
YY=/tmp/boinc_log-one_day-only_with_Authorization_required.txt && journalctl
--since=today --identifier=boinc > $XX && journalctl --since=today
--identifier=boinc --grep='Authorization required' > $YY && diff --unified=1
$XX $YY
…
@@ -118836,69 +118764,2 @@
…
-Oct 02 08:31:02 Sonne boinc[9863]: 02-Oct-2023 08:31:02 [---] Starting BOINC
client version 7.25.0 for x86_64-suse-linux-gnu
-Oct 02 08:31:02 Sonne boinc[9863]: 02-Oct-2023 08:31:02 [---] This a
development version of BOINC and may not function properly
-Oct 02 08:31:02 Sonne boinc[9863]: 02-Oct-2023 08:31:02 [---] log flags:
file_xfer, sched_ops, task, benchmark_debug, cpu_sched_debug, gui_rpc_debug
-Oct 02 08:31:02 Sonne boinc[9863]: 02-Oct-2023 08:31:02 [---] log flags:
http_debug, http_xfer_debug, idle_detection_debug, network_status_debug
-Oct 02 08:31:02 Sonne boinc[9863]: 02-Oct-2023 08:31:02 [---] log flags:
poll_debug, work_fetch_debug
…
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---] Setting up GUI
RPC socket
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---] [gui_rpc] Local
control only allowed
…
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---] Checking
presence of 133 project files
…
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 Initialization
completed
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---]
CLIENT_STATE::poll_slow_events() started
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---] Running CPU
benchmarks
…
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---]
HOST_INFO::user_idle_time() started
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---] xss_idle()
started
…
-Oct 02 08:31:04 Sonne boinc[9863]: 02-Oct-2023 08:31:04 [---] xss_idle()
XOpenDisplay(:0) call
 Oct 02 08:31:04 Sonne boinc[9863]: Authorization required, but no
authorization protocol specified
…


I got the impression finally that a return from this function call did not
happen here so far while the shown error message was logged with a remarkable
frequency.
Would you like to reconsider function implementation details any more?