The issue was solved.
I upgraded JIRA to new (7.5.0) version. After that JIRA Software could be updated with success. I noticed that my JIRA core version was 7.2.6 when JIRA Software version was 7.4.2.
I guess, somehow, previous upgrading was not completed.
...PermissionsStartOnly=true User=www-data Group=www-data ExecStart=/opt/jira/bin/startup.sh ExecStop=/opt/jira/bin/shutdown.sh TimeoutStartSec=120 TimeoutStopSec=600 PrivateTmp=true [Install] WantedBy...
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