Error with TikZ figure that complie well before

Question

I've got this diagram created with Tikz a year before:

\documentclass[paper=letter,headings=big,bibliography=totoc,DIV=9]{scrbook}
\KOMAoptions{headsepline=true,fontsize=10pt,draft=false}
\usepackage{typearea}
\usepackage[spanish.mexico]{babel}
\usepackage[utf8]{inputenc}
\usepackage[LGR,T1]{fontenc}
\usepackage{tikz}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}
\usetikzlibrary{through}
\usetikzlibrary{backgrounds}
\usetikzlibrary{fit}
\usetikzlibrary{fadings}

\begin{document}

\begin{figure}[!h]
\begin{center}
\begin{tikzpicture}
% Trazado de las placas paralelas
Placas: \coordinate (P1) at (0,0);
\coordinate (P2) at (8.5,0);
\coordinate (P3) at (0,1.2);
\coordinate (P4) at (8.5,1.2);
\coordinate (P5) at (0,-0.1);
\coordinate (P6) at (8.5,-0.1);
\fill[blue!20!white] (P1) -- (P3) -- (P4) -- (P2) -- cycle;
\draw (P1) -- (P2);
\draw (P3) -- (P4);
\fill[gray!80!white] (P1) -- (P5) -- (P6) -- (P2) -- cycle;
% Dimensión de la distancia t entre placas
Texto:     \node[draw=none,fill=blue!20!white] (t) at (1,0.5) {$\epsilon$};        
\draw[<->] (1.2,0) -- (1.2,1.2);   % line with error
Texto: \node[draw=none,fill=blue!20!white] (t2) at (2,0.5) {$fluido$};
Texto: \node[draw=none,fill=white,above] (t3) at (1,1.2) {placa móvil};
Texto: \node[draw=none,fill=white,below] (t2) at (1,-0.1) {placa fija};
% Volumen de control y deformación
\node[below] (a) at (3.3,0) {$a$};
\node[below] (b) at (4.8,0) {$b$};
\node[above] (c) at (4.8,1.2) {$c$};
\node[above] (d) at (3.3,1.2) {$d$};
\node[above] (c1) at (5.3,1.2) {$c'$};
\node[above] (d1) at (3.8,1.2) {$d'$};
\draw (a) -- (d)
(b) -- (c);
\draw[dashed] (a) -- (d1)
(b) -- (c1);
% Perfil de velocidades
\coordinate (u1) at (5.8,0);
\coordinate (u2) at (5.8,1.2);
\coordinate (u3) at (6.8,1.2);
\draw (u1) -- (u2) 
(u1) -- (u3);
% Vector U
\coordinate (u4) at (5.8,1.4);
\coordinate (u5) at (6.8,1.4);
\draw[->] (u4) -- (u5); % Line with error
Texto: \node[draw=none,fill=white,above] (U) at (6.3,1.4) {$U$};
% coordenadas
\coordinate (u6) at (5.8,1);
\coordinate (u7) at (6.6,1);
\coordinate (y1) at (6.6,0);
\draw[->] (u6) -- (u7) % Line with error
(y1) -- (u7); % Line with error
Texto: \node[draw=none,fill=blue!20!white] (u8) at (6.2,1) {$u$};
Texto: \node[draw=none,fill=blue!20!white] (y2) at (6.6,0.5) {$y$};
% ejes
\coordinate (x1) at (8.6,0);
\node (x2) at (9.1,0) {$x$};
\coordinate(y3) at (3.3,1.7);
\node (y4) at (3.3,2.5) {$y$};
\draw[->] (x1) -- (x2); % Line with error
\draw[->] (y3) -- (y4); % Line with error
% Flecha F
\coordinate (F1) at (7.7,1.4);
\coordinate (F2) at (8.5,1.4);
\draw[thick,->] (F1) -- (F2); % Line with error
Texto: \node[draw=none,fill=white,above] (U) at (8.1,1.4) {$F$};
\end{tikzpicture}
\end{center}
\caption[Deformación a cortante constante.]{Deformación resultante de la aplicación de una fuerza cortante constante.}
\label{fig:placas}
\end{figure}
\end{document}

But when I compile the lines commented as % Line with error shows the lines wrong, I don't understand why, before I compiled well this figure. The error I've got is:

Missing \endcsname inserted. \draw[thick,->]