Either VMware Standard or QuickSpin backups fail saying the virtual machine quiesce fails or that Changed Block Tracking (CBT) could not be used; this behavior begins to happen after upgrading to ESXi 6.0 or may happen if using ESXi 6.0 to begin with. In the source virtual machine's VMware logs, you may see messages similar to this:
vcpu-0| I120: DISKLIB-CBT : Creating cbt node 92b78c-cbt failed with error Cannot allocate memory (0xbad0014, Out of memory)
As per VMware's documentation "This issue occurs due to heap exhaustion. It can occur when attempting to enable Change Block Tracking (CBT). If a virtual machine with a large number of virtual disks reached an upper threshold, enabling CBT fails because of heap exhaustion. This issue also occurs with multiple virtual machines with CBT enabled. In case of Windows virtual machines with VSS enabled, taking a quiesced snapshot creates double the amount of memory overhead."
VMware reports this issue as being fixed in ESXi 6.0 Build 2715440. Patching your ESXi machine to that version or newer should alleviate the issue.