Are you having issues with screen sessions started as one user yet you have errors connecting from su to the user?
As it turns out, you can't. Well, probably without patching the code yourself :-)
If you have errors like this:
Cannot open your terminal '/dev/pts/3' - please check. (or '/dev/pts/0'...)
You will have to resort to logging in as the user to re-connect to the wayward screen session. line Thanks goes to www.houseofmorten.com
And as always, in case that link breaks:
run script /dev/null as the user you su to before launching screen