Two figure colums side by side, subfigure aligment issues

Question

I want to create two colums of figures according to the following MWE. My solution is inspired by:

How can I vertically align two minipages with subfigures and differing caption length?

I have to use "subfigure" instead of subfloat because of incompatibilities from any other approaches with various packages I need for my non-MWE document.

As you can see from the MWE-Output, the left column's figures are somewhat misaligned - is there any remedy to this??? I am missing the text "Testing a long text string" in the left colum - the left column figures seem to be placed too far to the right. This is what happens in my original document, too.

In my original problem, things look like this:

Clearly, the column(s) is/are somewhat misaligned...

Basically I copied the above cited solution - why do I nest all the stuff in a figure environment? I know that if I don't put all this in a figure-environment, the minipage/subfigure approach won't work out...

\documentclass[]{article} 


\usepackage{graphicx}                       % Grafikpaket (ermöglicht     unter anderem das Einbinden von .jpg-Bildern)
\usepackage{subcaption}
\usepackage{psfrag}
\usepackage{lscape}
\usepackage{color}                       % Color Paket wird benötigt, damit matlab2tikz-Farben richtig übersetzt werden

%pgfplots etc
\usepackage{todonotes}

\begin{document}

\begin{figure}
\begin{minipage}[t]{.48\textwidth}
\centering
\begin{subfigure}[b]{0.48\linewidth}
\missingfigure[figwidth=\textwidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{0.48\linewidth}
\missingfigure[figwidth=\textwidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{0.48\linewidth}
\missingfigure[figwidth=\textwidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}

\caption{First figure with two subfigures with very very very very long caption}
\label{fig:testa}
\end{minipage}\hfill     \begin{minipage}[t]{0.48\textwidth}
\begin{subfigure}[b]{0.48\linewidth}
\missingfigure[figwidth=\textwidth]{Testing a long text string}
        \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{0.48\linewidth}
\missingfigure[figwidth=\textwidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{0.48\linewidth}
 \missingfigure[figwidth=\textwidth]{Testing a long text string}
        \caption{efe}
\end{subfigure}
 \caption{Second figure with two subfigures}
\label{fig:testb}
    \end{minipage}
     \end{figure}


\end{document}

Answer 1

Link which you provide, show the way how to managede your images. Using example-image from package graphics and change subfigure and included image width that consider width of minipage you can obtain:

MWE for above image:

\documentclass[]{article}
\usepackage{graphicx}
\usepackage{subcaption}

\begin{document}
    \begin{figure}
\begin{minipage}[t]{.48\textwidth}
\centering
\begin{subfigure}[b]{\linewidth}
\includegraphics[width=0.9\hsize]{example-image}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\includegraphics[width=0.9\hsize]{example-image}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\includegraphics[width=0.9\hsize]{example-image}
    \caption{efe}
\end{subfigure}

\caption{First figure with two subfigures with very very very very long caption}
\label{fig:testa}
\end{minipage}
    \hfill     
\begin{minipage}[t]{0.48\textwidth}
\begin{subfigure}[b]{\linewidth}
\includegraphics[width=0.9\hsize]{example-image}
        \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\includegraphics[width=0.9\hsize]{example-image}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
 \includegraphics[width=0.9\hsize]{example-image}
        \caption{efe}
\end{subfigure}
 \caption{Second figure with two subfigures}
\label{fig:testb}
\end{minipage}
    \end{figure}
\end{document}

Answer 2

In your code, once you've entered a minipage, the widths become \linewidth, not 0.48\linewidth:

\documentclass[]{article}

\usepackage[showframe]{geometry} \usepackage{graphicx} % Grafikpaket (ermöglicht unter anderem das Einbinden von .jpg-Bildern)
\usepackage{subcaption}
\usepackage{psfrag}
\usepackage{lscape}
\usepackage{color} % Color Paket wird benötigt, damit matlab2tikz-Farben richtig übersetzt werden

%pgfplots etc
\usepackage{todonotes}

\begin{document}

\begin{figure}
\begin{minipage}[t]{.48\textwidth}
\begin{subfigure}[b]{\linewidth}
\centering
\missingfigure[figwidth=\linewidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\centering
\missingfigure[figwidth=\linewidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\centering
\missingfigure[figwidth=\linewidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}

\caption{First figure with two subfigures with very very very very long caption}
\label{fig:testa}
\end{minipage}\hfill \begin{minipage}[t]{0.48\linewidth}
\begin{subfigure}[b]{\linewidth}
\centering
\missingfigure[figwidth=\linewidth]{Testing a long text string}
        \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\centering
\missingfigure[figwidth=\linewidth]{Testing a long text string}
    \caption{efe}
\end{subfigure}\\
\begin{subfigure}[b]{\linewidth}
\centering
 \missingfigure[figwidth=\linewidth]{Testing a long text string}
        \caption{efe}
\end{subfigure}
 \caption{Second figure with two subfigures}
\label{fig:testb}
    \end{minipage}
     \end{figure}

\end{document}