"Error: Timeout reached while waiting for PID 212" while decrypting LUKS over SSH

I've been trying to open a LUKS container via SSH using dropbear and initramfs/initrd image. My setup works well. I mean, I log via SSH to the target machine, then I see the prompt saying Please unlock disk rpi_crypt: , and then I enter the right password and the LUKS container is being opened and the system boots. But when I typed the password via SSH, after a few secs I got the following error: $ ssh 192.168.1.239 Please unlock disk rpi_crypt: Error: Timeout reached while waiting for PID 212. Connection to 192.168.1.239 closed. So where does this error come from and how to fix it?