无法在ios8中的文档目录中复制可写plist文件

Am trying to copy a plist file which has a http url written to it from app bundle to documents directory but its getting failed only in ios8 with message "NSInternalInconsistencyException', reason: 'The operation couldn’t be completed. (Cocoa error 4.)'.'" .Its working fine in ios7.Please guide what could be the reason.Am trying to copy a plist file which has a http