Hi Timo,
Could you please add some additional informations (as root) :
* ldd /usr/lib/x86_64-linux-gnu/libgtkmm-3.0.so.1 | grep gtk
* find /usr/lib -name '*gtk*.so*' -exec nm --print-file-name --dynamic
{} \; | grep gtk_level_bar_get_type
* dpkg -l libgtk*
Thanks,
--
Léo