I recently upgraded to 18.04 and various things are broken. In particular, various applications give error messages when trying to launch:
firefox /usr/lib/firefox/firefox: symbol lookup error: /usr/lib/x86_64-linux-gnu/libgtk-3.so.0: undefined symbol: g_log_structured_standard inkscape inkscape: symbol lookup error: /usr/lib/x86_64-linux-gnu/libgiomm-2.4.so.1: undefined symbol: g_application_set_option_context_description gimp gimp: symbol lookup error: gimp: undefined symbol: g_object_new_with_properties gedit gedit: symbol lookup error: /usr/lib/x86_64-linux-gnu/libgtk-3.so.0: undefined symbol: g_log_structured_standard
Other things (Chrome, Sublime Text) work fine.
I'm using Xubuntu/xfce and it isn't working well either: none of the usual menu panels (whisker menu, power indicator, etc.) will work, and I can't login normally. On boot I get a loop between the usual startup screen and a black screen with a mouse, although I can switch to another tty and launch the GUI manually with startx.
It seems like some sort of graphics library didn't get upgraded properly, or maybe there's an old version hanging around. I'm not sure how to troubleshoot that myself.
EDIT: I looked through /var/log/dist-upgrade/[upgrade date] and nothing looks odd, although I don't really know what to look for.