Internal laptop screen stuck on Ubuntu loading screen, secondary monitor working just fine. (Ubuntu 18.04)

I did not have this issue myself, but here is an educated guess, informed by other cases of "stuck on the loading Ubuntu screen with the 5 dots":

The display manager (which should display the login screen) cannot start until the graphical "loading Ubuntu" screen has finished, which seems to not happen for your internal display. Since that graphical "loading Ubuntu" screen does not necessarily display on all available screens, that could be the reason why your external screen is unaffected.

A good indicator of this cause for the problem is when you have the following message in /var/log/boot.log:

A start job is running for Hold until boot process finishes up (Xmin Xs/no limit)

The graphical "loading Ubuntu" screen is called plymouth, and it is not essential at all. When it is not installed, a text-mode screen will show the five dots instead. So the following may already fix your issue:

sudo apt remove plymouth

And then reboot. For variants of this solution, some specifically from people with Nvidia driver issues, see this question.