The problem: Being root you would like to resume another user's screen session. Let the user be A.
You 'become' him by
su A -
or
sudo su - A
Then you would like to
screen -r
but unfortunately this gives you an error:
Cannot open your terminal '/dev/pts/0' - please check.
or something similar. The simple cause is that user A does not have the right to write into your (root's) terminal. This can be fixed by chmod'ing the above displayed file. For example:
chmod a+rwx /dev/pts/0
Do it as root and be aware that this is a severe security risk!
Modified from: http://highmem.blogspot.com/2006/07/how-to-resume-another-users-screen.html