Qt5 xcb-xlib backend fails to initialize

On a particular environment - which is a server machine that has the necessary X11 and xcb packages according to my knowledge - all Qt applications (for example all the examples) crashing in graphic backend initializing code. It happens either with X forwarding and by running the app with xvfb-run.

Output xvfb-run:
No platform plugin argument was specified, defaulting to "xcb".
Successfully connected to display :101

Information of screen 232:
width.........: 640
height........: 480
depth.........: 8
white pixel...: 1
black pixel...: 0

QIBusPlatformInputContext: ibus config file '/home/balazs/.config/ibus/bus/124cb6f3e543db1073af49560001668c-unix-0' does not exist.
QIBusPlatformInputContext: not connected.
no window!
Aborted

Backtrace with xvfb-run:
#0 0xf6c27537 in raise () from /lib/libc.so.6
#1 0xf6c2a922 in abort () from /lib/libc.so.6
#2 0xf6f80873 in qt_message_output (msgType=QtFatalMsg, buf=0x8143a00 "no window!") at global/qglobal.cpp:2208
#3 0xf6f80a32 in qt_message (msgType=QtFatalMsg, msg=0xf55839ec "no window!", ap=0xffffcfb4 "") at global/qglobal.cpp:2254
#4 0xf6f80d98 in qFatal (msg=0xf55839ec "no window!") at global/qglobal.cpp:2432
#5 0xf5563647 in QXcbWindow::create (this=0x813ebf0) at qxcbwindow.cpp:223
#6 0xf5562fca in QXcbWindow (this=0x813ebf0, window=0x80a8c80) at qxcbwindow.cpp:136
#7 0xf5555cf9 in QXcbIntegration::createPlatformWindow (this=0x8083668, window=0x80a8c80) at qxcbintegration.cpp:112
#8 0xf73c0c4a in QWindow::create (this=0x80a8c80) at kernel/qwindow.cpp:155
#9 0xf796256a in QWidgetPrivate::create_sys (this=0x80a2fb0, window=0, initializeWindow=true, destroyOldWindow=true) at kernel/qwidget_qpa.cpp:130
#10 0xf7928d3f in QWidget::create (this=0xffffd3ec, window=0, initializeWindow=true, destroyOldWindow=true) at kernel/qwidget.cpp:1379
#11 0xf79395ab in QWidget::setVisible (this=0xffffd3ec, visible=true) at kernel/qwidget.cpp:7343
#12 0x0806505a in QWidget::show (this=0xffffd3ec) at ../../../include/QtWidgets/../../src/widgets/kernel/qwidget.h:494

Output with X forward:
No platform plugin argument was specified, defaulting to "xcb".
Successfully connected to display localhost:13.0

Information of screen 349:
width.........: 2560
height........: 1024
depth.........: 24
white pixel...: ffffff
black pixel...: 0

Running window manager: KWin
QIBusPlatformInputContext: ibus config file '/home/balazs/.config/ibus/bus/124cb6f3e543db1073af49560001668c-localhost-13' does not exist.
QIBusPlatformInputContext: not connected.
Warning: no suitable glx confiuration found

Backtrace with X forward:
#0 0xf6eab2f5 in glXGetVisualFromFBConfigSGIX () from /usr/lib/libGL.so.1
#1 0xf557f72c in qglx_findVisualInfo (display=0x807e130, screen=0, format=...) at glxconvenience/qglxconvenience.cpp:162
#2 0xf556342d in QXcbWindow::create (this=0x8144010) at qxcbwindow.cpp:195
#3 0xf5562fca in QXcbWindow (this=0x8144010, window=0x80aa570) at qxcbwindow.cpp:136
#4 0xf5555cf9 in QXcbIntegration::createPlatformWindow (this=0x8083668, window=0x80aa570) at qxcbintegration.cpp:112
#5 0xf73c0c4a in QWindow::create (this=0x80aa570) at kernel/qwindow.cpp:155
#6 0xf796256a in QWidgetPrivate::create_sys (this=0x80a7860, window=0, initializeWindow=true, destroyOldWindow=true) at kernel/qwidget_qpa.cpp:130
#7 0xf7928d3f in QWidget::create (this=0xffffd40c, window=0, initializeWindow=true, destroyOldWindow=true) at kernel/qwidget.cpp:1379
#8 0xf79395ab in QWidget::setVisible (this=0xffffd40c, visible=true) at kernel/qwidget.cpp:7343
#9 0x0806505a in QWidget::show (this=0xffffd40c) at ../../../include/QtWidgets/../../src/widgets/kernel/qwidget.h:494
#10 0x08064e25 in main (argc=1, argv=0xffffd524) at main.cpp:162