Extra }, or forgotten $. ... z@ $relax $ seqstrict}}{39}{figure.4.7} and so on

This time I have a strange problem. Following code is from the *.lof file:

babel@toc {german}{}
addvspace {10p@ }
contentsline {figure}{numberline {1.1}{ignorespaces Architektur des $mathbb {K}$-Frameworks}}{3}{figure.1.1}
addvspace {10p@ }
addvspace {10p@ }
contentsline {figure}{numberline {3.1}{ignorespaces IMP-Modul}}{26}{figure.3.1}
contentsline {figure}{numberline {3.2}{ignorespaces Syntaxdefinitionen}}{26}{figure.3.2}
contentsline {figure}{numberline {3.3}{ignorespaces Endmodul}}{26}{figure.3.3}
addvspace {10p@ }
contentsline {figure}{numberline {4.1}{ignorespaces IMP-Konfiguration}}{31}{figure.4.1}
contentsline {figure}{numberline {4.2}{ignorespaces K-Regeln}}{33}{figure.4.2}
contentsline {figure}{numberline {4.3}{ignorespaces Beispielhafte Betrachtung eines Terms}}{34}{figure.4.3}
contentsline {figure}{numberline {4.4}{ignorespaces K-Konfigurationen in Backus-Naur-Form}}{35}{figure.4.4}
contentsline {figure}{numberline {4.5}{ignorespaces Regeln f"ur $relax $@@underline {hbox {tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}}}mathsurround z@ $relax +relax $@@underline {hbox {tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}}}mathsurround z@ $relax $ strict}}{37}{figure.4.5}
contentsline {figure}{numberline {4.6}{ignorespaces Rechenklasse f"ur en Ausdruck $xtmspace +thinmuskip {.1667em}*tmspace +thinmuskip {.1667em}(ytmspace +thinmuskip {.1667em}+tmspace +thinmuskip {.1667em}2)$}}{38}{figure.4.6}
contentsline {figure}{numberline {4.7}{ignorespaces Regeln f"ur $relax $@@underline {hbox {tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}}}mathsurround z@ $relax <=relax $@@underline {hbox {tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}tmspace +thinmuskip {.1667em}}}mathsurround z@ $relax $ seqstrict}}{39}{figure.4.7}
contentsline {figure}{numberline {4.8}{ignorespaces KResult und KResultLabel}}{39}{figure.4.8}
addvspace {10p@ }
addvspace {10p@ }

The lines with figure 4.5 and 4.7 are causing the errors:

Extra }, or forgotten $. ... z@ $relax $ seqstrict}}{39}{figure.4.7}
Extra }, or forgotten $. ...und z@ $relax $ strict}}{37}{figure.4.5}

I give you the problematic parts from my original tex file:

begin{align}
&a_{1}<=a_{2}rightleftharpoons a_{1}curvearrowright Box<=a_{2}
&r_{1}<=a_{2}rightleftharpoons a_{2}curvearrowright r_{1}<=Box
end{align}
begingroupvspace*{-baselineskip}
captionof{figure}{Regeln für $underline{,,,}<=underline{,,,}$ seqstrict}
vspace*{baselineskip}endgroup

begin{align}
a_{1},+,a_{2},rightleftharpoons,a_{1},curvearrowright,Box,+,a_{2}
a_{1},+,a_{2},rightleftharpoons,a_{2},curvearrowright,a_{1},+,Box
end{align}
begingroupvspace*{-baselineskip}
captionof{figure}{Regeln für $underline{,,,}+underline{,,,}$ strict}
vspace*{baselineskip}endgroup

I don’t see any mistakes there. Compiling works fine, i.e. PDF file is being generated and everything seems fine. But these errors are disturbing me. I don’t know what I am doing wrong. Maybe the align environment is incompatible with the capt-of package, this might cause the problem. But the others sections are working fine. All other figures excluding 4.5 and 4.7 have the same structure, but no errors.

Help me guys!

Thanks

documentclass{book}
usepackage[T1]{fontenc}
usepackage[ngerman]{babel}

usepackage{amsmath,amssymb}

newcommand{blank}{underline{mspace{9mu}}}

begin{document}

frontmatter

listoffigures

mainmatter

begin{figure}[htp]
setlength{abovecaptionskip}{0pt}

begin{align}
&a_{1}<=a_{2}rightleftharpoons a_{1}curvearrowright Box<=a_{2}
&r_{1}<=a_{2}rightleftharpoons a_{2}curvearrowright r_{1}<=Box
end{align}
caption{Regeln für $blank <= blank$ seqstrict}

begin{align}
a_{1},+,a_{2},rightleftharpoons,a_{1},curvearrowright,Box,+,a_{2}
a_{1},+,a_{2},rightleftharpoons,a_{2},curvearrowright,a_{1},+,Box
end{align}
caption{Regeln für $blank + blank$ strict}
end{figure}

end{document}

newcommand{blank}{protectunderline{mspace{9mu}}}