Most users who encounter this probably have an issue with the core storage volume; but for me the issue was that there were no local users able to unlock the disk. It took a while as there was not a a very helpful error or anything in the logs.
In my situation had deployed an image using DeployStudio which created the local admin user as a hidden user and was trying to get Profile Manager to work but kept getting stuck on the FileVault, none of the logs had anything useful in them, just things like
com.apple.preference.security.remoteservice CoreStorage conversion failed with mainError: -69886; detailError :0
The fix was to simply add a new local user and then enable it to unlock the disk.