I replaced the driver, now it works. But I think it might have been also something in slicer or configuration in marlin. I resetted it once again to make sure it's clean start.

Hi, Just to write what helped me with the z-wobble problem. First I changed the coupling to the aluminum - helped a bit. Than I added this "Makertech Axis 3D printer Z steady" (https://www.thingiverse.com/thing:4570666). It also helped but still there was this slightly visible wobble each 1cm of Z. Finally I reassembled the gantry and the lead screw minding the instructions. What was important, I think, in stage6/steps 9-12/, was to tighten each guide in the middle position of lead screw and I also tightened it a bit in the top and bottom position just to make sure. Only after that I fixed the screws on the motor.

Hi, I write just to say that I also tried to find the solution for dual print. For me it finally worked when I replaced the couplings with PC4-M5 bowden the brass version which seemed to have a bit longer thread. This allowed me to screw them into the splitter much tighter. I also widened each end of 1cm ptfe tubes (using a thin screwdriver) in the splitter so that the filament wouldn't get stuck. I also reassembled the hotend to make sure that there is no gap between the nozzle and the ptfe tube - that's because I've noticed that when the filament retracted it somehow got much wider on the /retracted/end - that's probably the other reason why it got stuck. I thing there must have been a gap in which it could get wider, cool down, get stuck. I also have a problem with that blob of filament when the nozzle exchanges the filament - no idea what to do.

Hi, Thank you very much for the answer! This step by step instructions should be handy any time I have problems with printer. I checked once again the mainboard visually, chcecked the cables if anything didn't go loose while printing. I didn't start to disassemble since it worked fine for few prints. And I found the source of malfunction. Its the A4988 Stepper Driver where the capacitor burned. I don't know why. But since there is a spare one in the box I could easily replace this one. Everything now works fine. Except dual printing but that's another case.. still the printer can't really replace the filaments correctly. Always one of them gets stacked. Thanks again. Just to show below there is an image of burned capacitor /dark smudge on the soldering/ on the driver.