P: Export As says "An unknown error occurred."

When I choose “Export as” from the file menu, or rightclick in the layers panel to export a layer, the window that pops up for exporting just says “An unknown error occurred” and I cannot select anything. 

I just updated to beta 27.4.0, and this seems to be happening on every document. This also only seems to be happening if I have “Use Legacy Export As” Selected under preferences? 

I need the legacy mode to be enabled considering Adobe made some weird changes to how their PNG8 exporting works at some point, I need the legacy version to preserve the alpha channel on those.
It still does Quick Export to PNG correctly, even if I have both “Legacy mode” and “export as 8bit” on the Quick Export as PNG option, so it at least seems like file type that isn’t part of the issue. No idea what is though.

Extra note, I saw someone with a similar problem where theirs freezes when they switch to JPG, so it might be good to note that my dialog window is defaulting to JPG momentarily before the error pops up.

OS: Windows 10

Photoshop version: Beta 27.4.0