Why do I get some error like
Paragraph ended before
\Hy@setref@linkwas complete
when I insert table of contents in book class documents and the error dissapear when I place a comment in \usepackage{hyperref}?
[Tex/LaTex] Problem with insert table of contents with hyperref
hyperref
Related Solutions
The backref package that is used for the pagebackref option changes catcodes within the bibliography to find the end of bibliography entries. This breaks all stuff that relies on certain catcodes like URLs, verbatim text, and DOIs. As explained in the backref manual the catcode changes can be suspended with \backrefparscanfalse, but then it is your responsibility to tell backref where your bibliography entries end, and the page list can be written. The following works without altering the DOI:
\documentclass{article}
\usepackage[pagebackref=true]{hyperref}
\usepackage{doi}
\begin{document}
\cite{fort:cl11}
\begin{thebibliography}{1}
\backrefparscanfalse
\bibitem[{Fort et~al.(2011)Fort, Adda, and Cohen}]{fort:cl11}
Fort, Karën; Gilles Adda; and K.~Bretonnel Cohen (2011).
\newblock {Amazon Mechanical Turk}: Gold mine or coal mine?
\newblock \emph{Computational Linguistics}, 37(2):413--420.
\newblock \doi{10.1162/COLI_a_00057}.
\backrefprint\backrefparscantrue
\end{thebibliography}
\end{document}
In the likely case that you don't really setup your thebibliography environment by hand, but with the help of BibTeX and some bibliography style, please see this answer how to change the bibliography style to automatically insert the \backrefparscanfalse and \backrefprint\backrefparscantrue command before and after bibliography items.
The fix is obtained by removing the \\ from the line, following the \ref.
As I commented, the use of \\ is specialized for things like tabular and should generally be avoided in normal text. See David's answer at When to use \par and when \\, or blank lines for a better discussion.
I don't know what hyperref did to actually make the code break by placing a \\ immediately following a \ref, but since it is bad LaTeX to use it, good grammar takes care of the problem.
As barbara adds, an equation environment is generally not preceded by a paragraph break.
And, as egreg adds, the "F" in the text should be $F$ to place it in math mode.
\documentclass{article}
\usepackage{hyperref}
\begin{document}
$F$ is defined in Formula \ref{eq:Fdef}
\begin{equation} \label{eq:Fdef}
F = RU
\end{equation}
\end{document}
Best Answer
This can happen when you have a faulty
.auxfile which is corrupted during a previous compile that could have had an error in it. Remove the faulty.auxfile, and recompile (at least twice).