Thanks for help.
Don't know why (and don't know if boinc-client.service is case sensitive) but I mistyped:
PrivateTMP=true

So repaired it to PrivateTmp=true and now am waiting for new tasks.


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My complete boinc-client.service is like this (should I uncomment or add something else?):

### Editing /etc/systemd/system/boinc-client.service.d/override.conf
### Anything between here and the comment below will become the new contents of the file

[Service]
PrivateTmp=true

### Lines below this comment will be discarded

### /lib/systemd/system/boinc-client.service
# [Unit]
# Description=Berkeley Open Infrastructure Network Computing Client
# Documentation=man:boinc(1)
# After=network-online.target
#
# [Service]
# Type=simple
# ProtectHome=true
# ProtectSystem=strict
# ProtectControlGroups=true
# ReadWritePaths=-/var/lib/boinc -/etc/boinc-client
# 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
# # The following options prevent setuid root as they imply NoNewPrivileges=true
# # Since Atlas requires setuid root, they break Atlas
# # In order to improve security, if you're not using Atlas,
# # Add these options to the [Service] section of an override file using
# # sudo systemctl edit boinc-client.service
# #NoNewPrivileges=true
# #ProtectKernelModules=true
# #ProtectKernelTunables=true
# #RestrictRealtime=true
# #RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX
# #RestrictNamespaces=true
# #PrivateUsers=true
# #CapabilityBoundingSet=
# #MemoryDenyWriteExecute=true
# #PrivateTmp=true #Block X11 idle detection
#
# [Install]
# WantedBy=multi-user.target

As a general rule of thumb, everything in Linux is case-sensitive.
It should be right just this way.
Eventually coming tasks will confirm it.