Removing snapshot causes Unexpected signal: 11. Most of the backup software relies on creating and removing snapshots.
Bug is not officially confirmed by VMWare. No fix, the only solution is to roll back to ESXi 5.5 update 2.
See the following post for more details:
https://communities.vmware.com/thread/520379
Update 1/10/2015 14:30 EST: VMware released kb http://kb.vmware.com/kb/2133118
Update 7/10/2015 15:30 EST: VMware released kb http://kb.vmware.com/kb/2133825