The POSTSNAPSHOTCMD command failed.
If the command specified by the POSTSNAPSHOTCMD option completed with a non-zero return code, the snapshot differential backup operation continues. The operation continues with a warning-level result. The result of the POSTSNAPSHOTCMD command does not supersede a higher result from the snapshot differential backup command. For example, if the snapshot differential backup command completed with code 12, a lower return code from the POSTSNAPSHOTCMD command does not alter the return code.
The snapshot differential backup continues, but with a warning return code of at least 8.
Identify and repair the problem that caused the command to fail. Consider wrapping the command in a script that always exits with a return code of zero.