A few reboots later, I'm now able to unlock the notebook from the LightDM lockscreen. There's no "switch to greeter" anymore, but a real password entry field which works. However, I still don't know why I had the GDM3 lockscreen before.
A few reboots later, I'm now able to unlock the notebook from the LightDM lockscreen. There's no "switch to greeter" anymore, but a real password entry field which works. However, I still don't know why I had the GDM3 lockscreen before.