\mathplus fails to render

If you can not remove this (rather pointless) definition as it is being auto-generated, put

\newcommand\mathplus{+}

in your document preamble so that \mathplus expands to +.

You should use unicode-math package by adding:

  \usepackage{unicode-math}

in the beginning of your file.

remember to delete all auxiliary files including the .bbl file then run your .tex file using XeLaTeX and again using BibTeX and again using XeLaTeX.