Login: When I start Xfce a dialog pops up saying "Could not look up internet address for..."

Xfce simply wants your hostname to be in /etc/hosts. Example input: 127.0.0.1 localhost

Logout: When I try to log out by pressing the logout button in the panel, I get a dialog asking me whether I want to quit the panel and/or xfce4-session-logout reports that no session manager is running, but it is!

For some reason, your X applications can not connect to the session manager. Possible causes are: your hostname cannot be resolved (see Login problems section), your home partition or partition containing /tmp is filled up, your hostname contains non-ascii characters (no umlauts allowed, in particular) or that either ~/.ICEauthority or /tmp/.ICE-unix has wrong permissions. Also check .xsession-errors for clues.