Graphics drivers update and try to change output in the settings

What should I change the output to?

Try whether one works, it's not too many and the conf file gives you the possibilities. But update your graphics drivers first

I tried changing the outputs, but that didn't do anything.

Dominus wrote:

But update your graphics drivers first

My graphics driver is up-to-date (since I just bought this computer a month ago), but when I rolled back, I was able to play in fullscreen. However, the previous driver doesn't support my resolution, and DOSBox was working just fine with the fully updated driver last month, so it seems pretty clear that the graphics driver isn't necessarily at fault here.

It seems pretty clear that there is a problem with your graphics driver IMO. DID you check whether there is a new driver for your graphics card?

My graphics driver is up-to-date

I don't see why it would suddenly stop working when I can't recall doing anything to either DOSBox's display settings or my graphics card. Although I did have some trouble fenangling with the Duke 3D High-Res Pack, I fixed that by adding "-opengl" when I ran it. Can't remember what I may have done in regards to updating the graphics, though.

Then both the up-to-date and rolled-back drivers are flawed.

When you have the option to roll back drivers you DID change them since you got the machine. You can only roll back when there was a change 😀
But feel free to ignore advice and find another fix.

I had a sudden burst of insight and remembered that I changed DOSBox's fullscreen output from something to surface. Changed it to overlay (I don't know if this is what it was before) and it worked!

So your system is definitely borked.