If you get the following errors starting screen as a user:

No more PTYs.
Sorry, could not find a PTY.
[screen is terminating]

You have to change /dev/ptmx file permissions:

# ls -l /dev/ptmx 
crw-r--r-- 1 root root 5, 2 nov 18 23:28 /dev/ptmx
# chmod a+w /dev/ptmx
# ls -l /dev/ptmx 
crw-rw-rw- 1 root root 5, 2 nov 18 23:34 /dev/ptmx

Under Ubuntu, there was no problem. Permissions already set:

$ ls -l /dev/ptmx 
crw-rw-rw- 1 root tty 5, 2 2009-11-18 23:37 /dev/ptmx

References :

  • man ptmx
  • ubuntu$ ls -l /dev/ptmx