VM cannot be backup with an existing Snaphot

I'am going crazy, anytime i try to backup a VM with an existing Snapshot it fails

I try to Backup an VM from a ESXI-Testserver with ESXI Version 5.1.0 build-838463.

The value from the variable ALLOW_VMS_WITH_SNAPSHOTS_TO_BE_BACKEDUP is 1.

Dryrun Log: http://pastebin.com/R9ubPns1

Debug Log: http://pastebin.com/BqnPa9Wi

If there is no existing Snapshot, the backup works fine.

I have the same problem on another ESXI Server with ESXI Version 5.0.

Any Idea what is wrong?