God damn, the line
Qt Code: 
  1. qDebug() << query.lastError().text();    // debug
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returns
Qt Code: 
  1. warning: "No database selected QMYSQL: Unable to execute query"
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. Jacek, thanks for hints, but clear this up for me please. How is it possible that in the same object and in other method, one time the database is active and the second time, like in this problem, it goes offline. Is there any timeout setting for MySQL database???