Note on https://dev.mysql.com/doc/refman/5.7/en/libmysqld.html says: “The libmysqld embedded server library is deprecated as of MySQL 5.7.17 and will be removed in MySQL 8.0.“

Ok guy thank you for that much replies :P! I solved the issue by myself anyway. In case sombebody has a similar problem here comes the solution:

What I did wrong was that I connected a time consuming function to the UpdateClient::tcpReady() slot. In this function I did some stuff which also lead to the emission of UpdateClient::tcpReady(). This broke the QEventQueue. So to what I had to chance was to skip the direct signal slot connection an do this in the UpdateClient::tcpReady():

QTimer::singleShot(0, this, SIGNAL(updateAvailable()));

No everything is working fine ;).

No, I close connection after try to connect and after conditional statement, as it is in the code in first post.