Thank you, Lykurg, for your help.
No, I have not set the local of the MS SQL Server yet. I have already tried QString::fromUtf8() but there is no good results.
The only case when everything works fine is when one has e.g. "QByteArray encode = "\05E4\05E2\05E9";". But in my database there are "????" instead of characters and they are on my widgets hence.
Anyway, I will try to follow your advices, thanks again.