Hmm, yea. However, I have no idea what buffer it could be. Each file is ~400 KB PNG image. I always open QFile and put it using QFtp or QNetworkAccessManager. When put gets finished I close QFile and then do this again with another file. I have tried running app with GDB but it does not give me any meaningful information. I guess I just have to try to Google more and more. Anyway, thanks. And if you would have any other idea, please let me know.