I was having an issue when I upgraded to Cura 3.4.1 and I figured some other people might, so I just wanted to put this out there.
Each time I started a print, my Z probe would hit the bed as my printer was going from doing the bed levelling and actually printing the part. The bed levelling would complete and the X-carriage would move to X200 and successfully retract the probe. However, the next move after this would be to X0 Y0 Z15 and it would cause the probe to extend again and hit the bed.
I figured out the error was the Start G-code. After the bed leveling command (G29) was “G1 Z15 F6000”, which tells the probe to go to Z15 (meant to avoid collisions i think). Now when this command was compiled by Cura, it changed to “G1 X0 Y0 Z15 F6000”, which told the printer to move to X0 Y0 Z15 everytime after the bed levelling was completed. This obviously caused the Zprobe to extend and hit the bed.
To fix this, all you have to do is edit this command to something else. Go to Preferences > Configure Cura > Printers > Machine Settings. Edit the command to say “G1 X15.0 Y15.0 Z15.0 F6000”. Then slice a part, and open the gcode file in a text editor to see if the command changed. This should now make your printhead move to X15 Y15 Z15 at the start of every print instead of X0.