What did the timer below the preview say in the production window?
Did you get any hint on an error?
Can you check the target drive? Is there enough space on it? What is the file system? It should be anything but a version of FAT. You can check by right clicking on the drive and clicking on "properties". There it should list the file system being used.
Also try to uncheck the OpenCL and hardware decoding options and render without GPU acceleration or SVRT, to be sure the error isn't there.
Does the encode always stop at the same time for the same project? If yes, is there something special going on on the time line, like a new media file starting that needs a different codec?
That's just what I would check if it was happening to me.

Quote: What did the timer below the preview say in the production window?
Did you get any hint on an error?
Can you check the target drive? Is there enough space on it? What is the file system? It should be anything but a version of FAT. You can check by right clicking on the drive and clicking on "properties". There it should list the file system being used.
Also try to uncheck the OpenCL and hardware decoding options and render without GPU acceleration or SVRT, to be sure the error isn't there.
Does the encode always stop at the same time for the same project? If yes, is there something special going on on the time line, like a new media file starting that needs a different codec?
That's just what I would check if it was happening to me.

So for some reason, I just tried exporting it again and it came out fine. I didn't change any settings. I'm not sure what caused it the last two times.
Would running a game while the video exports cause that?

Sure, that's possible when both PD and the game use the GPU and the gfx driver has a bug. But it could just as well not be related to the game.