Check accelerated helper
If you're unexpectedly in fallback mode, the first thing to do is to check whether the gnome-session-check-accelerated helper does:
/usr/libexec/gnome-session-check-accelerated; echo $?
If this prints "1", an error message should also be printed.
The ~/.xsession-errors file is the place where some components and applications log messages. However, if you are failing to log in, it can be painful to extract the data, because it's normally overwritten on subsequent login. Currently then:
- Try to log in from GDM and encounter an error
- Switch to a virtual console with control-alt-F2 and log in there
- Run: cp ~/.xsession-errors ~/.xsession-errors.crash
- Log out