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Well I found one thing that caused an error:
\end{sol}\end{ug}

writing it as
\end{sol}
\end{ug}
solved the problem.

I must have forgotten some of all I've learnt about latex, but I thought that having them on the same line would not matter.
I was obviously wrong or have I missed something ...

Hello.
I've just gotten an error with my latex file. I get

! TeX capacity exceeded, sorry [main memory size=3000000].
\mathsm@sh #1#2->\setbox \z@ \hbox {$\m@th #1{#2}$
}\finsm@sh
l.27933 ...rightarrow{a}=\lambda\overrightarrow{b}
$.
If you really absolutely need more capacity,
you can ask a ...