Can not attach to running screen session

I have ubuntu 14.04 LTS version and there is a screen session. The session is alive, it has a program running which is generating files. I can also see it on the process list (ps aux):

sampie 1377 0.0 0.0 87140 1736 ? Ss marras03 0:00 SCREEN -R -d

However, I am unable to attach to that session.
"screen -list" command says: "No Sockets found in /var/run/screen/S-sampie."

"screen -r" says: "There is no screen to be resumed."

I don't know if this is a bug or not, but how can I attach to the existing screen session?

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: screen 4.1.0~20120320gitdb59704-9
ProcVersionSignature: Ubuntu 3.13.0-39.66-generic 3.13.11.8
Uname: Linux 3.13.0-39-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.5
Architecture: amd64
Date: Thu Nov 20 10:44:01 2014
InstallationDate: Installed on 2014-07-08 (135 days ago)
InstallationMedia: Ubuntu-Server 14.04 LTS "Trusty Tahr" - Release amd64 (20140416.2)
SourcePackage: screen
UpgradeStatus: No upgrade log present (probably fresh install)