Development on Qt in KDE - standard dialog boxes do not work, why?

Question

Qt

Hello!
I am developing an application in QT. It calls the standard dialog boxes of the form

QFileDialog::getExistingDirectory(0, "Directory Dialog", "");

The application works fine on Windows, Linux (Unity, MATE, XFCE, LXDE).
However, I work with KDE. And here, when the dialog box is called, strange things begin to happen. The window and the whole program completely freezes when the dialog is called.
This is what output says:
When the application starts

kf5.kiconthemes: "Theme tree: (Breeze)"

And here is what happens when the dialog box is first displayed.

QInotifyFileSystemWatcherEngine::addPaths: inotify_add_watch failed: No such file or directory
kf5.kservice.sycoca: Trying to open ksycoca from "/home/aa/.cache/ksycoca5"

Probyval any examples to do - all the same. Why is this in KDE? Is there any fine tuning required.
Googled, the problems were the same only when developing for Android.
ps Another glitch. Clicked on some button in your dialog box, an error is immediately displayed:

load glyph failed err=6 face=0x9c85c10, glyph=964

And in this window, the processing of all signals suddenly stops.
Please help with the problem, I thought that there should be no problems with Qt on KDE at all. However, it turned out the other way around - on all popular desktop interfaces under Linux everything works fine as it should, but in KDE everything is somehow not stable.
Trying to reinstall QT without any addons did not help.