Qsqlquery: problem with multiple queries at the same time.

[spruzzer]

I wrote some widgets which I instantiate in a main window. Every widget is bound to a database using a class called dbManager, which implements a qtsqldatabse object and some functions to insert into the db tables. The dbManager cass is passed as reference to every widget in order to have a singe database connection. When I'm inserting record into the db (a threaded Class) and I want to browse the tabe content I get this message:

Unable to free statement: lost synchronization with server: got message type "?", length -33673258

and the program crash.

To load data into the db entries browser I use this pattern to update the shown data:

@ QSqlQuery query;
query.prepare("SELECT T.content FROM tweet T WHERE T.topic = :topic AND LANGUAGE = :language AND NOT EXISTS (SELECT V.tweet_id FROM voted_tweet V WHERE T.tweet_id = V.tweet_id)");
query.bindValue(":topic",arg1);
query.bindValue(":language",ui->langSelector->currentText());
query.exec();
queryModelcol->setQuery(query);
ui->tableView->setModel(queryModelcol);

    //  --- Full Model
    QSqlQuery query2;
    query2.prepare("SELECT * FROM tweet T WHERE T.topic = :topic AND LANGUAGE = :language AND NOT EXISTS (SELECT V.tweet_id FROM voted_tweet V WHERE T.tweet_id = V.tweet_id)");
    query2.bindValue(":topic",arg1);
    query2.bindValue(":language",ui->langSelector->currentText());
    query2.exec();
    queryModelrow->setQuery(query2);@

In the DB manager cass records are inserted in the folowing way:

@QSqlQuery query;
query.prepare("INSERT INTO TWITTER_USER (user_id, name, language, img)"
"VALUES (:user_id, :name, :language, :img)");

query.bindValue(":user_id", data.at(0).toUtf8().constData());
query.bindValue(":name", data.at(1).toUtf8().constData());
query.bindValue(":language", data.at(2).toUtf8().constData());
query.bindValue(":img", data.at(3).toUtf8().constData());
if (query.exec() )
    return true;
else
    return false;@

I'm using PostgreSql with the psql driver on Windows. I tryed to set transactions for every query execution and also to use a single query object with .finish() function, but the result is Always the same.
Do you have suggestions to find a way to solve the error?