Comment on How to work around screen “Cannot open your terminal ‘/dev/pts/1’ – please check.” by matthias.
you can work around it using ssh. just put the public key of your user or root into the target user .ssh
you would probably want to use a ssh agent script so you only need to auth your private key once over multiple shells and dont have to fiddle with ssh-agent manually.
then you can just use ssh in replacement for su. so you ssh user@localhost
then you can easily use screen cause ssh opens another pty that really belongs to the according user.