XCB error: 5 (BadAtom)

[Ecke]

When starting a software (Flameshot - I'm a new maintainer) compiled with Qt 5.15.0 I get:

[martin@martin-laptop src]$ ./flameshot
qt.qpa.xcb: QXcbConnection: XCB error: 5 (BadAtom), sequence: 568, resource id: 0, major code: 20 (GetProperty), minor code: 0

once at startup.

Does anyone know more?
Can't find any documentation regarding this.
Sounds like an XCB problem that Qt does not specify further and just print?

Manjaro 20.3, xfce4, xorg-server 1.20.8-4
Laptop with APU and dedicated graphics card ( Acer Aspire V 15)
(will test on other devices)

Have two external monitors one via VGA one via DP.
(same error without them)

Thanks for your time in advance!