Probe Basic main screen doesn't show loaded toolpath

I just installed to test and no issue with the vtk as you describe so I'm going to venture it's a compatibility issue, can you provide your machine specifications and your video card and driver and openGL version.

Intel(R) Core(TM)2 Quad CPU    Q9400  @ 2.66GHz
4gb RAM

NVIDIA Corporation C79 [GeForce 9300 / nForce 730i] (rev b1)
Kernel driver in use: nouveau
Kernel modules: nouveau
OpenGL version string: 3.3 (Compatibility Profile) Mesa 18.3.6

in axissim gcode-path shows up normally

tried nvidia drivers, which killed my system, had to reinstall linuximage missing /media/kunena/emoticons/ermm.png

axis uses a different backplotter that is not compatible with qtpyvcp, qtpyvcp uses vtk for backplotting and there are some known compatibility issues but it was previously thought to only be with openGL versions older than 1.5. I also know that rpi's have some vtk compatibility issues.

during the installation did you run the line in step 3? in terminal?

sudo apt install python-pyqt5 python-pyqt5.qtquick python-dbus.mainloop.pyqt5 python-pyqt5.qtopengl python-pyqt5.qsci python-pyqt5.qtmultimedia qml-module-qtquick-controls gstreamer1.0-plugins-bad libqt5multimedia5-plugins pyqt5-dev-tools python-dev python-wheel python-setuptools python-pip git python-pyqtgraph python-pyqt5.qtwebkit

and did it succeed or have errors?

there were no errors.
just checked again:

git ist schon die neueste Version (1:2.20.1-2+deb10u3).
gstreamer1.0-plugins-bad ist schon die neueste Version (1.14.4-1+deb10u2).
pyqt5-dev-tools ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-dbus.mainloop.pyqt5 ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-pyqt5 ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-pyqt5.qtmultimedia ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-pyqt5.qtopengl ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-pyqt5.qtquick ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-pyqt5.qtwebkit ist schon die neueste Version (5.11.3+dfsg-1+b3).
python-dev ist schon die neueste Version (2.7.16-1).
python-pip ist schon die neueste Version (18.1-5).
python-pyqtgraph ist schon die neueste Version (0.10.0-1).
python-setuptools ist schon die neueste Version (40.8.0-1).
python-pyqt5.qsci ist schon die neueste Version (2.10.4+dfsg-2.1).
libqt5multimedia5-plugins ist schon die neueste Version (5.11.3-2).
qml-module-qtquick-controls ist schon die neueste Version (5.11.3-2).
python-wheel ist schon die neueste Version (0.32.3-2).
0 aktualisiert, 0 neu installiert, 0 zu entfernen und 145 nicht aktualisiert.

can you try changing the backplot preview by selecting X View on the left side of the plotter window.  wondering if it may be a refresh issue.

what gcode file are you trying to load?  is it an example or one of your own? is the issue the same with the example files?

i tried my own gcode ,created gcode fom conversational and the arcspiral from pb_examples.
pressing x-view and all other buttons doesnt let the loaded path appear