Possible runaway argument using circuitikz

The main point is made by siracusa: the parser gets confused by the [...] which are nested in the trouble-making line. This can be resolved by making the respective coordinate a symbolic coordinate via

\path ([xshift=10mm]nmos9.drain) coordinate (aux); 

In addition you cannot have spaces around -- in kinky cross=.... This is because it is not an ordinary path, where spaces would have been indeed allowed, but some style defined via style args.

\documentclass[tikz]{standalone}
\usepackage[compatibility]{circuitikz}
\ctikzset{bipoles/length=1cm}

\tikzset{
    declare function={% in case of CVS which switches the arguments of atan2
    atan3(\a,\b)=ifthenelse(atan2(0,1)==90, atan2(\a,\b), atan2(\b,\a));},
    kinky cross radius/.initial=+.125cm,
    @kinky cross/.initial=+, kinky crosses/.is choice,
    kinky crosses/left/.style={@kinky cross=-},kinky crosses/right/.style={@kinky cross=+},
    kinky cross/.style args={(#1)--(#2)}{
        to path={
            let \p{@kc@}=($(\tikztotarget)-(\tikztostart)$),
            \n{@kc@}={atan3(\p{@kc@})+180} in
            -- ($(intersection of \tikztostart--{\tikztotarget} and #1--#2)!%
            \pgfkeysvalueof{/tikz/kinky cross radius}!(\tikztostart)$)
            arc [ radius =\pgfkeysvalueof{/tikz/kinky cross radius},
            start angle=\n{@kc@},
            delta angle=\pgfkeysvalueof{/tikz/@kinky cross}180 ]
            -- (\tikztotarget)
        }
    }
}

\begin{document}
    \begin{tikzpicture}[every text node part/.style={align=right}]
        \def\fourthlx{9}
        \node[nmos, xscale=-1] (nmos7) at (\fourthlx,0) {};
        \node[nmos, xscale=-1] (nmos8) at (\fourthlx,1.1) {};
        \node[nmos, xscale=-1] (nmos9) at (\fourthlx,2.2) {};
        \node[circ] (topresreplica) at ([yshift=15mm] nmos9.drain) {};
        \draw (topresreplica.center) to[*R] ([yshift=2mm] nmos9.drain) -- (nmos9.drain);

        \def\fifthlx{15.5}
        \node[nmos] (nmos10) at (\fifthlx,0) {};
        \node[nmos] (nmos11) at (\fifthlx-1,1.1) {};
        \node[nmos, xscale=-1] (nmos12) at (\fifthlx+1,1.1) {};
        \node[nmos, xscale=-1] (nmos13) at (\fifthlx-1,2.2) {};
        \node[nmos] (nmos14) at (\fifthlx+1,2.2) {};
        \node[circ] (topres1) at ([yshift=15mm] nmos13.drain) {};
        \node[circ] (topres2) at ([yshift=15mm] nmos14.drain) {};
        \draw (topres1.center) to[*R] ([yshift=2mm] nmos13.drain) -- (nmos13.drain);
        \draw (topres2.center) to[*R] ([yshift=2mm] nmos14.drain) -- (nmos14.drain);

        \draw (nmos7.gate) -- (nmos10.gate);
        \draw let \p{A} = ([xshift=20mm] nmos7.gate), \p{B} = (nmos9.drain) in
            node (temp) at (\x{A},\y{B}) {}
            ($(\p{A})!0.5!(temp)$) to[*R] (\p{A}) node[circ] {}
            (temp) to[*C] ($(\p{A})!0.5!(temp)$)
            (temp) to[*short,-*] (\p{B})
        ;

        \draw (nmos11.source) to[*short,-*] (nmos10.drain) to[*short] (nmos12.source);
        \draw (nmos13.gate) -- (nmos14.gate);

        \node (errtest) at ([yshift=8mm] nmos9.gate) {};
        \path ([xshift=10mm]nmos9.drain) coordinate (aux);
        \draw ($(nmos13.gate)!0.5!(nmos14.gate)$) to[*short,*-] ([yshift=8mm] $(nmos13.gate)!0.5!(nmos14.gate)$) to[kinky cross=(nmos13.drain)--(topres1.center), kinky crosses=left] (errtest.center)
        (errtest.center) to[kinky cross=(nmos9.drain)--(aux), kinky crosses=right] (nmos9.gate);

    \end{tikzpicture}
\end{document}

Just for fun, I redrew the entire circuit without using a single xshift or yshift.

\documentclass[tikz]{standalone}
\usepackage[compatibility]{circuitikz}
\ctikzset{bipoles/length=1cm}

\tikzset{
    declare function={% in case of CVS which switches the arguments of atan2
    atan3(\a,\b)=ifthenelse(atan2(0,1)==90, atan2(\a,\b), atan2(\b,\a));},
    kinky cross radius/.initial=+.125cm,
    @kinky cross/.initial=+, kinky crosses/.is choice,
    kinky crosses/left/.style={@kinky cross=-},kinky crosses/right/.style={@kinky cross=+},
    kinky cross/.style args={(#1)--(#2)}{%
        to path={
            let \p{@kc@}=($(\tikztotarget)-(\tikztostart)$),
            \n{@kc@}={atan3(\p{@kc@})+180} in
            -- ($(intersection of {\tikztostart}--{\tikztotarget} and #1--#2)!%
            \pgfkeysvalueof{/tikz/kinky cross radius}!(\tikztostart)$)
            arc [ radius =\pgfkeysvalueof{/tikz/kinky cross radius},
            start angle=\n{@kc@},
            delta angle=\pgfkeysvalueof{/tikz/@kinky cross}180 ]
            -- (\tikztotarget)
        }
    }
}

\begin{document}
    \begin{tikzpicture}[every text node part/.style={align=right}]
        \def\fourthlx{9}
        \node[nmos, xscale=-1] (nmos7) at (\fourthlx,0) {};
        \node[nmos, xscale=-1] (nmos8) at (\fourthlx,1.1) {};
        \node[nmos, xscale=-1] (nmos9) at (\fourthlx,2.2) {};
        \path (nmos9.drain) ++(0,15mm) coordinate (topresreplica);
        \draw (nmos9.drain) -- ++(0,2mm) to[*R,-*] (topresreplica);

        \def\fifthlx{15.5}
        \node[nmos] (nmos10) at (\fifthlx,0) {};
        \node[nmos] (nmos11) at (\fifthlx-1,1.1) {};
        \node[nmos, xscale=-1] (nmos12) at (\fifthlx+1,1.1) {};
        \node[nmos, xscale=-1] (nmos13) at (\fifthlx-1,2.2) {};
        \node[nmos] (nmos14) at (\fifthlx+1,2.2) {};
        \node[circ] (topres1) at (nmos13.drain |- topresreplica) {};
        \node[circ] (topres2) at (nmos14.drain |- topresreplica) {};
        \draw (nmos13.drain) -- ++(0,2mm) to[*R] (topres1.center);
        \draw (nmos14.drain) -- ++(0,2mm) to[*R] (topres2.center);

        \draw (nmos7.gate) -- (nmos10.gate);
        \path (nmos7.gate |- nmos9.drain) -- ++(20mm,0) coordinate (topthingy);
        \path (nmos7.gate -| topthingy) coordinate (bottomthingy);
        \draw (nmos9.drain) to[*short,*-] (topthingy)
          to[*C] ($(topthingy)!0.5!(bottomthingy)$)
          to[*R,-*] (bottomthingy);


        \draw (nmos11.source) to[*short,-*] (nmos10.drain) to[*short] (nmos12.source);
        \draw (nmos13.gate) -- (nmos14.gate);

        \path (nmos9.gate) ++(0,8mm) coordinate (errtest);
        \draw ($(nmos13.gate)!0.5!(nmos14.gate)$) coordinate (midgate)
          to[*short,*-] (errtest -| midgate)
          to[kinky cross=(nmos13.drain)--(topres1), kinky crosses=left] (errtest)
          to[kinky cross=(nmos9.drain)--(topthingy), kinky crosses=right] (nmos9.gate);

    \end{tikzpicture}
\end{document}