# Missing \endcsname inserted error

I am getting an error that I have identified to stem from my bib file for one particular entry. This error has popped up after I moved from a natbib and BibTeX workflow to BibLaTeX and biber.

I had multiple citations causing this error which I have identified to have been stemming from % being in the bibliography which had to be escaped like so \%. However, the problem has persisted for a citation that does not seem to have any such special character as listed here. I have looked here but I cannot find any hidden character using this tool.

main.tex contents:

\documentclass[margin=2cm,varwidth]{standalone}

\usepackage[backend=biber,sorting=nty,style=apa,backref=true]{biblatex}

\begin{document}

\cite{Casey2017}
\printbibliography

\end{document}


Terminal Output:

line 45: Missing \endcsname inserted. \cite{Casey2017}
line 49: Missing \endcsname inserted.
: Rerun to get /PageLabels entry.
line 49: Overfull \hbox (20.45493pt too wide) in paragraph
: There were undefined references.
: Please (re)run Biber on the file:(biblatex) main(biblatex) and rerun LaTeX afterwards.


main.bib with only the offending bibliography:

@Article{Casey2017,
author     = {Casey, Michael},
journal    = {Texas Conference on Digital Libraries (TCDL), 2017, Austin, Texas, United States},
title      = {Why {Media Preservation Can't} {Wait:} {The Gathering Storm}},
year       = {2017},
month      = may,
abstract   = {This presentation explores the time-based media preservation problem, detailing how the forces of degradation along with rapidly advancing obsolescence issues taken together result in a short time window in which to digitally preserve audio, video and film collections. Using a fairytale as a framework, it introduces a character named Degralescence that embodies the major forces that threaten to undermine media preservation efforts. It also proposes a timeframe within which action must be taken to avoid losing the opportunity to affordably preserve large holdings of media recordings. Using the Indiana University collections and the work of the IU Media Digitization and Preservation Initiative as examples, it explores strategies for addressing this impending and inconvenient crisis. Finally, the presentation highlights international progress with an overview of key projects and resources.},
file       = {:https\://tdl-ir.tdl.org/bitstream/2249.1/82129/1/casey_tcdl_20170518.pdf:application/pdf},
groups     = {Analysis Methods},
language   = {en\_US},
shorttitle = {Why {Media} {Preservation} {Can't} {Wait}},
timestamp  = {2021-05-05},
url        = {https://tdl-ir.tdl.org/handle/2249.1/82129},
urldate    = {2021-05-05},
}


Any help is appreciated.

  language   = {en_US},

The braces in the title field seem wrong. The capitalization of titles should be left to the bibliographic style and only words that must keep capitalization should be braced.
• That said, en_US is not a particularly helpful language value either (and if it ends up being printed, it may cause an error): The language given in the language field should either be the literal name of the language or (preferably) the localisation key for the language (one of the languages listed in §4.9.2.18 Language Names, where the lang prefix can be dropped). Jun 1 at 18:16
• The underscore in the language is what was causing the problem. JabRef insists on the colon being escaped for the URL. Jun 1 at 19:50