The bottom part of the bolt is 'anchor' shaped see the poster above iminhell \_I_/
For now get some pie plate material, 'drill' a hole then cut/shape it so it covers the 'anchor'.
The other possibility is that you have some sort of anti corrosive felt pad around the post-dump it
Anyway now tighten the connection, it should work
If it does then hunt down some thin brass sheet and replace that first piece
To eliminate galvanic corrosion of dissimilar metals
Congrats of having a multimeter, SO MANY here think they can just look at stuff and solve electrical problems and start throwing parts at the situation