MySQL update solved the issue.
Many thanks for your help.
yes of course,
MySQL, server version 5.5.61-38.13-log, client version mysqlnd 7.4.3.
a detail may probably be useful for you: we have a third level domain with the same database, theme, and the matomo plugin aswell, but the theme version is not the latest; in our main site the problem occurred after the matomo update and the theme was updated just a few days before.
