I have cross compiled Qt 5.15.2 for Rasperry Pi 4 by following this instruction: https://wapel.de/?p=842

After finishing all steps of the instruction, I started an example project with qt widgets.
I run the Rasperry Pi 4 in Desktop Mode. The ssh connection and the deployment of the project is working well, but there is no application window on the Rasperry Pi 4. I just got the following error description in Qt Creator:

Failed to move cursor on screen HDMI1: -13
Failed to move cursor on screen HDMI1: -13
Could not set cursor on screen HDMI1: -13
failed to export dumb buffer: Permission denied
Failed to create scanout resource
failed to export dumb buffer: Permission denied
Failed to create scanout resource
QEGLPlatformContext: eglSwapBuffers failed: 3003
Could not lock GBM surface front buffer!

The same problem occured in this thread (which is still unsolved):
https://forum.qt.io/topic/116911/eglfs-full-screen-issue-on-raspberry-pi-4-desktop

How can I solve this problem?
Are there any forum members, who had the same issues?

Thank you in Advance!