Update May 14
VMware release a bugfix for this issue: Patch ESXi-6.0.0-20150504001-standard (2116127)
Backup of virtual machines running on vSphere 6.0 can fail when changed block trackng (CBT) is enabled. This is because heap exhaustion which results in No memory available! errors and failed backup. The issue occurs on a virtual machine with a large number of virtual disks.
Arcserve UDP 5.0 Update 4 has a workaround which does a full backup of a virtual machine if CBT enabled backup fails. Read the Arcserve blog about this issue here.
VMware is working on a Express Patch 2 for ESXi 6.0 but no details on the release date. VMware advise is to disable Change Block Tracking in the virtual machine.