Has this VM been uploaded from e.g. VMware Workstation? The file format of the VM-...-s00x.vmdk files (2GB sparse) is not supported by ESXi 5.1.
What I guess is that these files are not in use and due to their existence the snapshot creation fails. However, to be sure it's necessary to see a complete list of files (names, sizes, time stamps) in the VMs folder and check which .vmdk files are in use by taking a look at the VM's configuration (.vmx) file as well as its latest vmware.log.
If you cannot provide the information/files for security reasons, check whether a descriptor .vmdk file for the VM-000001-s002x.vmdk files exists. This file should have the name VM-000001.vmdk and a size of a few hundred bytes. If it doesn't exist and the files are not mentioned in the .log file, the VM-000001-s00x.vmdk files are most likely obsolete. What you may do in this case is to create a sub-directory and move these VM-000001-s00x.vmdk files to it (with the VM powered off). Alternatively create an empty dummy VM-000001.vmdk file, this way the Snapshot Manager will chose another file name for the snapshot.
André