HyperFAIL

\documentclass{article}

\usepackage{answers}
\usepackage{enumitem}
\usepackage{hyperref}
\usepackage{multicol}
\usepackage{changepage}
\usepackage{etoolbox}

\newcommand\getcurrentref[1]{%
 \ifnumequal{\value{#1}}{0}
  {??}
  {\the\value{#1}}%
}

\hypersetup{colorlinks=true}

\setlength{\parindent}{0.0mm}


% very useful during de-bugging!
%\usepackage[left]{showlabels}
%\showlabels{hypertarget}
%\showlabels{hyperlink}

% solutions files
% short solutions
\Opensolutionfile{shortsolutions}
\Newassociation{shortsolution}{shortSoln}{shortsolutions}
\Newassociation{short}{shortSolns}{shortsolutions}
% long solutions
\Opensolutionfile{longsolutions}
\Newassociation{longsolution}{longSoln}{longsolutions}
\Newassociation{longs}{longsSolns}{longsolutions}

% new environment that sets up hypertargets both in the question
% section, and in the answer section
\newlist{myenum}{enumerate}{3}
\newcounter{question}[subsection]
\newenvironment{question}[1]%
    {%
    \refstepcounter{question}%
    %  hyperlink to solution
         \hypertarget{question:{\thesubsection\thequestion}}{}%
         \Writetofile{shortsolutions}{\protect\hypertarget{shortsoln:\thesubsection\thequestion}{}}%
         \Writetofile{longsolutions}{\protect\hypertarget{longsoln:\thesubsection\thequestion}{}}%
         \begin{myenum}[label=\bfseries\protect\hyperlink{shortsoln:\thesubsection\thequestion}{\thequestion.},ref=\thequestion]
         \item%
            \hypersetup{linkcolor=black}%
            \hyperlink{longsoln:\thesubsection\thequestion}{#1}%
        }%
        {%
        \end{myenum}}

\newenvironment{IntList}[1]{%
\Opensolutionfile{mysolutions}
\centering
\setlength{\columnsep}{50pt}
\begin{adjustwidth}{-3em}{-2em}\begin{multicols}{#1}}{\end{multicols}\end{adjustwidth}\Closesolutionfile{mysolutions}}


\begin{document}

\section{Problems}

\subsection{Questions}
\begin{multicols}{3}

                 \begin{short}
                \vspace*{\baselineskip}
                \color{white} hello
                \vspace*{-\baselineskip}
            \end{short}
            \begin{longs}
                 \vspace*{\baselineskip}
                \color{white} hello
                \vspace*{-\baselineskip}
            \end{longs}

    \begin{question}{$\int\sqrt{4-x}\mathrm{d}x$}
            \begin{shortsolution}
                $-\frac{2}{3}(4-x)^{\frac{3}{2}}+C$
            \end{shortsolution}
            \begin{longsolution}
                In order to do this, we must first make a substitution\ldots
            \end{longsolution}
    \end{question}
    \begin{question}{$\int(2x-1)^{50}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{1}{102}(2x-1)^{51}+C$
            \end{shortsolution}
            \begin{longsolution}
                A very long solution| much longer than the shorter solution
            \end{longsolution}
    \end{question}
    \begin{question}{$\int\sqrt[n]{x}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{n}{n+1}x^{\frac{1+n}{n}}+C$
            \end{shortsolution}
            \begin{longsolution}
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
                Even longer!
            \end{longsolution}
    \end{question}
\end{multicols}

\subsection{Some more questions}

\begin{multicols}{3}

                 \begin{short}
                \vspace*{\baselineskip}
                \color{white} hello
                \vspace*{-\baselineskip}
            \end{short}
            \begin{longs}
                 \vspace*{\baselineskip}
                \color{white} hello
                \vspace*{-\baselineskip}
            \end{longs}

    \begin{question}{$\int\sqrt{4-x}\mathrm{d}x$}
            \begin{shortsolution}
                $-\frac{2}{3}(4-x)^{\frac{3}{2}}+C$
            \end{shortsolution}
            \begin{longsolution}
                hi
            \end{longsolution}
    \end{question}
    \begin{question}{$\int(2x-1)^{50}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{1}{102}(2x-1)^{51}+C$
            \end{shortsolution}
            \begin{longsolution}
                ho
            \end{longsolution}
    \end{question}
    \begin{question}{$\int\sqrt[n]{x}\mathrm{d}x$}
            \begin{shortsolution}
                $\frac{n}{n+1}x^{\frac{1+n}{n}}+C$
            \end{shortsolution}
            \begin{longsolution}
               lets go
            \end{longsolution}
    \end{question}

\end{multicols}

\begin{huge}
Input files / include files here
\end{huge}

% close solution files
\Closesolutionfile{shortsolutions}
\Closesolutionfile{longsolutions}


\clearpage
\section{SHORT answers}
% renew the SHORT solution environment so that it hyperlinks back to
% the question
\renewenvironment{shortSoln}[1]{%
         % add some glue
         \vskip .5cm plus 2cm minus 0.1cm%
         {\bfseries \hyperlink{question:#1}{#1.}}%
}%
{%
}%
% input the file if it exists
\IfFileExists{shortsolutions.tex}{\input{shortsolutions.tex}}{}

\clearpage
\section{LONG answers}
% renew the LONG solution environment so that it hyperlinks back to
% the question
\renewenvironment{longSoln}[1]{%
         % add some glue
         \vskip .5cm plus 2cm minus 0.1cm%
         {\bfseries \hypersetup{linkcolor=blue}\hyperlink{question:#1}{#1.}}%
}%
{%
}%
% input the file if it exists
\IfFileExists{longsolutions.tex}{\input{longsolutions.tex}}{}
\end{document}