Bug#251602: oops! "fix" introduces a worse bug
Apparently changing "exec" to "." is not the right thing: it makes all
session types execute .xsession (or rather whatever the default behaviour).
Acting on my previous suggestion, it appears that things exit in
/etc/X11/Xsession.d/20xfree86-common_process-args (as sourced by
/etc/X11/Xsession) when the "which $1" (i.e. "which custom") command
fails (due to `set -e' at the beginning of /etc/X11/Xsession). This
appears not the intent of the script (it appears that the programmer
wanted something like
STARTUP_FULL_PATH=$(which $1 || true)
That aside, it now looks to me that the proposal of removing Custom from
the session menu is the best thing.