Measuring 19V at the tip of the lead gives you the off-load voltage from the power supply. Once you plug it in it comes under load and you would expect a drop to somewhere between 17 - 18V.
The 0.4V you quote could have two causes, only one of which involves the mobo. This one could be a faulty / broken motherboard connector (where the jack goes in) and this is by no means uncommon but can sometimes be repaired by careful use of a soldering iron. Some laptops have the socket connected to the mobo by a cable and these are easy to repair - just replace the socket.
The other possible fault, which you don't seem to have considered, is a faulty DC unit. If this has cooked, and it can happen, it could still give you normal off-load voltage but drop to virtually nothing when it comes under load.
I would check this first because it is much easier to do than pulling out a mobo ! Substitution is the easiest way to test. See if you can find somebody with a similar Acer laptop and try your power supply in their laptop and theirs in yours. Failing that, you will have to rig up some sort of dummy load by getting a resistor across the tip. A 220 to 560R should do the job nicely.