Well, that's lucky. So what I want you to do is this:
- Boot into the new kernel that isn't working
- Blindly log into LUKS and wait until you're pretty sure when you would have seen your login manager (GDM/LightDM/SDDM)
- Shut off the computer using the power button (NOT a hard shutdown, let it turn itself off)
- Boot back in but using your older working kernel
- Log back in as normal
- From a terminal run the following command
sudo journalctl -b-1 > bad-boot.log
- And the following
sudo journalctl -b0 > good-boot.log
- Verify that the timestamps in the log entries looks correct for each boot.
- Upload both files to hastebin or a similar paste service and share the links here.
This should hopefully give us enough to debug the issue.