! TeX capacity exceeded, sorry [input stack size=5000]

Question

My file was running fine, and I've since added a reference and the error (see title) has arrived. I had to remove the reference to fix it (editing the .bib file didn't help). I added another (different) reference today, and the same error came up.

I think it may be to do with the hyperref? It keeps coming up with a warning about it. I have an image in text with [H] to position it correctly.

MWE (I think it's important packages are seen):

\documentclass[12pt,a4paper]{report} 
\usepackage[round]{natbib}
\usepackage{color}
\usepackage{graphicx} 
\usepackage{siunitx}
\usepackage{hyperref}
\usepackage{float}
\setcounter{secnumdepth}{5}

\begin{document}                      
\maketitle
\tableofcontents
\newpage

\begin{figure}[H]
\centering
{\includegraphics[width=0.9\textwidth,natwidth=610,natheight=550]{blah}
\caption{blah blah blah}
\end{figure}

\bibliographystyle{plainnat}
\bibliography{LATEX2}

\end{document} 

I've tried removing the added reference, I've tried renaming the .bib file, checking the file paths, checking for special characters, removing URLs on recently added references... Not sure what else to do? It also comes up with this now: "! ==> Fatal error occurred, no output PDF file produced!" TIA.