Reading through some news blogs I found a very critical information for all customers thinking about upgrading to vSphere 6. No matter if vSphere 6 is called the best tested release ever, it seems that some cirtical bugs were still included. Unfortunaltely the CBT bug is one of them.
Keep that in mind if you want to use vSphere 6 in the near future or better wait for the patch to solve this issue. Patches for vSphere 5.x are already available.
It seems that the CBT bug in vSphere 6 is not exactly the same as it was under vSphere 5.x and below. This time, the bug will not hit you if you expand vmdk disks but rather it will hit you as soon as you have a "undetermined" amount of virtual disks that use CBT. VMware updated the KB2114076
and states there "If a virtual machine with a large number of virtual disks reached an upper threshold, enabling CBT fails because of heap exhaustion". But this is not only related to VMs with high number of disks but also: "This issue also occurs with multiple virtual machines with CBT enabled.". As the maximum number of disks per VM is still 60 this is probably the absolut maximum of disks with CBT enabled you can have before you run out of heap space and thus are not able to do CBT enabled snapshots anymore.
This is a ridiculous low number of disks you can protect under vSphere 6 so this is really a showstopper for deploying vSphere 6 at the moment. With update date of this article (05/07/2015) there is still no fix available.