Unwanted extra spaces are included before and after backslashes in biblatex URL

Question

I have a biblatex reference with a URL field. However, every backslash is surrounded by spaces.

I am using

\usepackage[sorting=nyt, citestyle=authoryear, bibstyle=authoryear, isbn=false, firstinits=true, url=false, terseinits=true, dashed=false, maxcitenames=3, maxbibnames=99, backref=true, doi=false]{biblatex}

What causes this, and how can I remove them?

EDIT: MWE

My biblatex .bib file is this:

@ONLINE{Soderberg2010,
author = {Soderberg, T},
title = {Section 5.1: The origin of the {NMR} signal.},
year = {2010},
url = {http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__5%3A_Structure_Determination_II/Section_5.1%3A_The_origin_of_the_NMR_signal},
urldate = {2013-5-16}
}

This works:

\documentclass[11pt]{amsart}
\usepackage{geometry}                % See geometry.pdf to learn the layout options. There are lots.
\geometry{letterpaper}                   % ... or a4paper or a5paper or ... 
%\geometry{landscape}                % Activate for for rotated page geometry
%\usepackage[parfill]{parskip}    % Activate to begin paragraphs with an empty line rather than an indent
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{epstopdf}
\DeclareGraphicsRule{.tif}{png}{.png}{`convert #1 `dirname #1`/`basename #1 .tif`.png}
\usepackage{multicol}

\usepackage[sorting=nyt, citestyle=authoryear, bibstyle=authoryear, isbn=false, firstinits=true, url=false, terseinits=true, dashed=false, maxcitenames=3, maxbibnames=99, backref=true, doi=false]{biblatex}
\addbibresource{resourcename}


\title{Brief Article}
\author{The Author}
%\date{}                                           % Activate to display a given date or no date

\begin{document}
\maketitle
%\section{}
%\subsection{}

\begin{multicols*}{2}
\parencite{Soderberg2010}
\end{multicols*}{2}

\printbibliography

\end{document} 

This doesn't work:

\documentclass[11pt]{amsart}
\usepackage{geometry}                % See geometry.pdf to learn the layout options. There are lots.
\geometry{letterpaper}                   % ... or a4paper or a5paper or ... 
%\geometry{landscape}                % Activate for for rotated page geometry
%\usepackage[parfill]{parskip}    % Activate to begin paragraphs with an empty line rather than an indent
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{epstopdf}
\DeclareGraphicsRule{.tif}{png}{.png}{`convert #1 `dirname #1`/`basename #1 .tif`.png}
\usepackage{multicol}

\usepackage[sorting=nyt, citestyle=authoryear, bibstyle=authoryear, isbn=false, firstinits=true, url=false, terseinits=true, dashed=false, maxcitenames=3, maxbibnames=99, backref=true, doi=false]{biblatex}
\addbibresource{resourcename}


\title{Brief Article}
\author{The Author}
%\date{}                                           % Activate to display a given date or no date

\begin{document}
\maketitle
%\section{}
%\subsection{}

\begin{multicols*}{2}
\parencite{Soderberg2010}
\end{multicols*}{2}

\begin{multicols*}{2}
\printbibliography
\end{multicols*}{2}

\end{document}

The only difference is that the second document has the reference list inside multicols.

Answer 1

biblatex gives some small stretchability in URL breaking using url package \Urlmuskip parameter. This is there whether or not multicolumn is used, it's just that in the narrow measure of a multi column setting the stretchability is more likely to be used if available.

There is probably a higher level biblatex setting for this, but this just patches the setup command to reset the parameter to zero.

adding this to your preamble

\toks0\expandafter{\biburlsetup}
\edef\biburlsetup{\the\toks0 \Urlmuskip =0mu\relax}

produces

Answer 2

I discovered that the problem was due to multicolumn. My solution was to disable multicolum for the reference list. If anyone has a solution that doesn't require disabling multicolumn for the reference list I will be very interested.