Well… the headline said everything, basically…
I had problems with my server and had to drive to it to unlock it, after a power-failure.
That was annoying, so i followed this guide (https://www.cyberciti.biz/security/how-to-unlock-luks-using-dropbear-ssh-keys-remotely-in-linux/) to be able to unlock it!
Works like a charme. The only downside is that the signature of the server changes so i have to clean the known-host line every reboot.
Apart from that its quite nice.
After connecting you simply have to type
and then your password.