The reason for this issue is indeed FileVault.
If you are comfortable using the Terminal there is a solution:
https://macminicolo.net/blog/files/restart-a-remote-mac-that-is-running-filevault-2.html
From Macminicolo:
When a Mac is using Filevault and restarts, it requires a password or recovery key before it will even start the bootup process into OS X. (…) Mountain Lion 10.8.2 introduced authrestart. If you open terminal and run “fdesetup help” then you will see all the options with filevault.
Now, when you need to restart a remote Mac that is running Filevault 2, you will open terminal and type “sudo fdesetup authrestart”. After doing this, you’ll be prompted for your password. Once entered, the machine will restart and bring you back to the OS login. It’s a one time restart. It’s a great tool.
If you are not comfortable using the Terminal the only solution is to disable FileVault:
The third option would be to boot from an unencrypted external drive (which does not make much sense, I guess).