Can't shut off Filevault


When I try to shut off FileVault, after logging out it says that an error occured while copying files and that the folders will continue to be encrypted. I have entered command + S at start up and performed a /sbin/fsck -fy and the disk is now OK. But I still can't shut off filevault, it gives the same message. Can anybody help me with this problem?
Thank you!