The file .xsession-errors in your home directory contains anything written to Standard Error by GUI-based programs launched from a desktop environment. If such a program crashes or fails to start, it can be useful to look at the last few lines in this file to see if any messages shed light on the problem:
Note this only applies to GUI programs launched from a desktop environment (e.g. from desktop menus or icons). GUI-based programs launched from a terminal session will normally write their errors back to that terminal session, unless you redirect them.