Not sure if helpful, but I had similar issue - machine would not boot if (only) using external monitors. My computer is a Dell Latitude 5590; it had previously booted, upon power up, with the machine closed, thus using only external monitors.
In order to diagnose, I started to roll back updates, and struck lucky the first time: I rolled back my BIOS (which had been updated by Ubuntu), and then it all Just Worked like it used to.
Not sure if helpful, but I had similar issue - machine would not boot if (only) using external monitors. My computer is a Dell Latitude 5590; it had previously booted, upon power up, with the machine closed, thus using only external monitors.
In order to diagnose, I started to roll back updates, and struck lucky the first time: I rolled back my BIOS (which had been updated by Ubuntu), and then it all Just Worked like it used to.
Not sure if related, but hopefully helpful.