Is the gray border around the mask apart of the mask? If so, recommend changing it to like a rgb (red, green, blue) color rather than gray. (not entirely sure if the gray would cause any issues for the mask but just a suggestion to try!)
Like Vortex said, it does indeed look like the sizing may be off for the Mask as well, maybe try making a new blank "mask" and setting the size to 513x513 and name it something different from "BorkMask.png" then export.
If none of that helps, maybe try changing it from .png to .jpg and see if that could help?
Also be sure to double check if you happen to have layers that they are all merged into one layer and not separate as I've had issues where my mask has crashed due to there being layers when I export! (can simply hide them or merge them either works!)
~ Hope this helps some!!