That's the cause:
You
- end a line, but not the paragraph,
- end a line again without beginning it at all,
- still nothing at the line, end you end it again, adding 48 pt space. All in the same paragraph.
\\
is the LaTeX command for ending a line, such as within a tabular, an array, a matrix, or a multi-line math environment. It is not for faking a vertical space which a line would take, this would be
\vspace{\baselineskip}
. Users often make the mistake thinking it could be like a "\skipline". Even more bad, it's all in the same paragraph, since it doesn't end the paragraph, so paragraph-wide settings for line spread and justification apply.
This "Underfull \hbox" error comes because TeX thinks in boxes. And there was a box (line) which wasn't filled with anything.
A proper way would be not inserting space by hand, but to design the document so it would not be necessary. It's often too hard and a user would like to manually tweak the output, but then a
\vspace
command should be used. An own macro (with
\vspace
inside) is better than a specific value, for document wide adjustments at such places.
Stefan