Check the PCB Design Manager. Look for an Incomplete Net warning then look at the lower part of the panel to check the pads or pins that the net connects to. That should highlight the node that the ratline is joining. You may need to zoom in to try to see the object. If all else fails, backup the project then select only the PCB as defined by the board Outline, excluding the area containing whtever the ratline is lead from. Then do CTRL-A (Select All) then delete. Then do CTRL-V to paste the PCB back into the PCB Editor. Save.

Thanks alot, your second advice solved the problem. Today arrived my order from JLCPLCB-Assembly-Service and it worked right away.

Deep joy. :)