This needs urgent investigation by community members with access to Apple hardware. Anyone who is still on 10.9, or who can downgrade to 10.9, do some tests, and upgrade to 10.10 again, please get in touch. Especially if you use Server.app.
If there are people using a Mac with VMware Fusion, note that you can create a VM running OSX by installing it from the recovery partition, something useful for tests. No need to switch definitely the main machine. -- Michael