Aus der Man-Page von xgterm:
ZitatBUGS
Many of the same bugs affecting xterm also apply here.Xgterm is not normally installed with setuid permissions. On some
Linux systems, for example, where the /dev/tty and /dev/pty devices
have root ownership and permission 600 this can cause problems.
Workarounds are to either install XGterm with setuid permissions or
modify the /dev/tty and /dev/pty devices to have permission 666.
hth,
JustAnotherUser