sidenotes package : error in figure alignment

MWE

\documentclass[12pt]{scrbook}

\usepackage{blindtext}

\usepackage{sidenotes}
\usepackage{mwe}

\usepackage[showframe]{geometry}
\geometry{paperwidth=170mm, paperheight=240mm, left=42pt, top=40pt, textwidth=280pt, marginparsep=20pt, marginparwidth=100pt, textheight=560pt, footskip=40pt}
%

\begin{document}

%
\begin{marginfigure}%
\includegraphics[width=\marginparwidth]{example-image-a}%
\caption{A small rectangle put in the margin.\label{rectangle}}%
\end{marginfigure}%
%
\blindtext
%
\begin{figure*}[htbp]
%\centering
\includegraphics[height=180pt,width=350pt]{example-image-c}%
\caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.}
\label{rectangle3}
\end{figure*}
%
\clearpage
%
\begin{marginfigure}%
\includegraphics[width=\marginparwidth]{example-image-a}%
\caption{A small rectangle put in the margin.\label{rectangle2}}%
\end{marginfigure}%
%
\blindtext
%
\begin{figure*}[htbp]
%\centering
\includegraphics[height=180pt,width=350pt]{example-image-c}%
\caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.}
\label{rectangle31}
\end{figure*}
%
\end{document}


Error in second page

You are a victim of end of line space. Put a % after [htbp]

\begin{figure*}[htbp]%
%\centering
\includegraphics[height=180pt,width=350pt]{example-image-c}%
\caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.}
\label{rectangle31}
\end{figure*}%


Full code:

\documentclass[12pt]{scrbook}

\usepackage{blindtext}

\usepackage{sidenotes}
\usepackage{mwe}

\usepackage[showframe]{geometry}
\geometry{paperwidth=170mm, paperheight=240mm, left=42pt, top=40pt, textwidth=280pt, marginparsep=20pt, marginparwidth=100pt, textheight=560pt, footskip=40pt}
%

\begin{document}

%
\begin{marginfigure}%
\includegraphics[width=\marginparwidth]{example-image-a}%
\caption{A small rectangle put in the margin.\label{rectangle}}%
\end{marginfigure}%
%
\blindtext
%
\begin{figure*}[htbp]
%\centering
\includegraphics[height=180pt,width=350pt]{example-image-c}%
\caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.}
\label{rectangle3}
\end{figure*}
%
\clearpage
%
\begin{marginfigure}%
\includegraphics[width=\marginparwidth]{example-image-a}%
\caption{A small rectangle put in the margin.\label{rectangle2}}%
\end{marginfigure}%
%
\blindtext
%
\begin{figure*}[htbp]%
%\centering
\includegraphics[height=180pt,width=350pt]{example-image-c}%
\caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.}
\label{rectangle31}
\end{figure*}%
%
\end{document}