I'm uploading a document to arXiv, but I have this rather cryptic error from the arXiv LaTeX compiler.

! Not loaded. \selectlanguage ...ing it}\errmessage {Not loaded} l.102 ...ad to divergences in {2D} diffusion}},\ } \href {https://doi.org/10.... ? ! Emergency stop. \selectlanguage ...ing it}\errmessage {Not loaded} l.102 ...ad to divergences in {2D} diffusion}},\ } \href {https://doi.org/10.... ! ==> Fatal error occurred, no output PDF file produced! Transcript written on main.log. [verbose]: pdflatex 'main.tex' failed. 

How can I debug this error? \selectlanguage is not in my source, but I'm struggling to understand from this message if that is the real source of the error.