Biblatex Error: Missing \begin{document} [duplicate]

I am trying to add a bibliograph to my document but I keep encountering the same error. My mwe is:

\documentclass{article}
\usepackage{biblatex}
\begin{document}
\cite{Lettenmeier.2017}
\end{document}


And I get the following error:

! LaTeX Error: Missing \begin{document}.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
...

l.3776   australian,newzealand}

?


I checked the biblatex.sty and the lite it refers to is:

\DeclareCaseLangs{%
english,USenglish,UKenglish,
australian,newzealand}


I already tried adding the babel package:

\usepackage[american]{babel}


But I get the same error. Does anyone have a clue what I might be doing wrong?

My refs.bib file looks like:

@article{Lettenmeier.2017,
author = {Lettenmeier, P. and Kolb, S. and Sata, N. and Fallisch, A. and Zielke, L. and Thiele, S. and Gago, A. S. and Friedrich, K. A.},
year = {2017},
title = {{Comprehensive investigation of novel pore-graded gas diffusion layers for high-performance and cost-effective proton exchange membrane electrolyzers}},
pages = {2521--2533},
volume = {10},
number = {12},
issn = {1754-5692},
journal = {{Energy {\&} Environmental Science}},
doi = {\url{10.1039/C7EE01240C}}
}


Cheers.

marked as duplicate by moewe biblatex StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Apr 24 '18 at 19:22

• Update the etoolbox package. See github.com/plk/biblatex/issues/706 and tex.stackexchange.com/q/413898/35864 – moewe Apr 24 '18 at 15:31
• doi = {\url{10.1039/C7EE01240C}} should just be doi = {10.1039/C7EE01240C} but that does not cause the error in question. The double braces in title = {{Comprehensive investigation of novel pore-graded gas diffusion layers for high-performance and cost-effective proton exchange membrane electrolyzers}}, are useless and should be avoided. The double braces in journal = {{Energy {\&} Environmental Science}}, are not completely useless, but don't do anything in your example - and rarely at all. – moewe Apr 24 '18 at 15:33
• I updated the etoolbox package with the same MWE and now latex prints whatever is inside the brackets of \cite, prints square brackets and makes it bold e.g. if instead of \cite{Lettenmeier.2017} i write \cite{foobar}it will print: [foobar] and ignore the refs.bib file. – elporsche Apr 24 '18 at 15:47