Yes, "no lines out of point 73" - you can't have a white dot that
isn't on the grid, and doesn't have a line, or colour edge, leading
it back to the grid. You should be able to locate the points in the
error message via a display somewhere in your editor which shows the
XY coordinates of your cursor on the image, ie 1770, 2318 and 2343, 2184
and see what is being complained about.

I often put unconnected dots on the grid, when I'm doing heightmapping,
when a layer on my stack is ASTER height data, converted to BoB height
map format, to capture inflection points in the height - ridgetops and
valley bottoms - as they pass across the grid. It is a quick and easy
form of heightmapping. But if I need to capture a hilltop with a white
dot, in the middle of the tile, then I must connect the dot to some
existing border or other, which leads back to the grid, via a blue fake line.

You have to think of mtt2 as a robot that crawls along the grid, and only
takes excursions into the tile when it encounters a line or colour edge
which crosses the grid with a white dot. Then it follows along the line,
and every line that breaks out along it, cataloguing the locations and
heights of all the dots it finds along the way, til it hits the grid again.
At that point it goes back to the first point on the grid past the one it
was following before it began the excursion, and continues along the grid
til it hits the next crossing line, then repeats. When it has gone round
the whole tile, it scans the tile for any white dots it hasn't encountered,
and puts them up as errors. This is the point where it can find a series of
white dots linked together but not connected to the grid, and it will commence
going around the series looking for an exit, indefinitely, and you have to
ctrl-C to kill it.