Yes, not only is it intermitent, but it does not happens in all the hosts with the same binary.
Maybe it would be better to try and move up into a newer qt release?

As we speak, I've moved to the point where I got no connection to any slots, nor do I have encapsulated nor added code in a new db manager class or such.

The code to add a new invoice is all encapsulated in a function, who in turn might use some static functions coming from an utility unit which also take in the db connection (so that, for example, I can choose if I save the invoice even though I could not register the stock differences, or a table that is for inspection that gets added a new row everytime something in the stock of products changes), but I do not connect to any signal/slot from QSqlQuery, nor from QSqlDatabase