I3 requires to read config which is in /home root user has a different config file that is not in /home but in /root
If you encrypted disk and /home when you unlock at boot you are only unblocking the disk but not /home
Probably i3 freezes because it can't read config file and also all the other important files like .bash .bashrc .bash_profile and everything else.
Make sure /home is decrypted before startx
