When latex fails to make a pdf-file out of your latex-file it can be hard to find the error in the latex-file.
Use of [fragile] on ordinary frames results in
Runaway argument?
! File ended while scanning use of \next.
<inserted text>
\par
<*> undervisning_med_R.tex
I suspect you have forgotten a `}', causing me
to read past where you wanted me to stop.
I'll try to recover; but if the error is serious,
you'd better type `E' or `X' now and fix your file.
! Emergency stop.
<*> undervisning_med_R.tex
*** (job aborted, no legal \end found)