Looks like my x-axis has too much friction with the metal bar, it's even eating into the bar and little metal pieces fall down. This problem became obvious after I've now finished the assembly and even started to solve initial problems with the extruder, so I have a completely assembled but kind of not working 3d printer now.
Any ideas how I can can fix the x-axis, e.g. make it move easier without having to disassemble the whole printer? Of course I "played" with the eccentric nuts that were used at the top, but I still have the same issues.
Y/Z move just fine...