Known problems when installing xfce4-session 4.13 from source:
Installing xfce4-session from source possibly may conflict with the distribution package installation of xfce4-session 4.12, leading to a crash during startup. When starting the self build xfce4-session from console, the following error is given: Gtk-Error **: GTK+ 2.x symbols detected. Using GTK+ 2.x and GTK+ 3 in the same process is not supported
It looks like the cause of the problem is, that libgtk-x11-2.0 and libgdk-x11-2.0 were linked.
However it is not fully clear why, and how it can be prevented (uninstall gtk2-dev ? )
it looks like the problem can be solved by removing xfce4-session which was installed by the distribution, re build from source and re-installing the self build xfce4-session. Note that on debian based systems it is required to use “apt-get –purge xfce4-session”. –remove is not sufficient.
Here some more information on it: https://bugzilla.xfce.org/show_bug.cgi?id=15104