Can't login or upload through Electron browser in PostyBirb

I have been trying to upload to the site through PostyBirb, which uses its own built-in Electron-based browser to access and post to the site.

The first problem is that I cannot log in through this browser at all. Even with the correct credentials, it tells me that my login info is incorrect. The problem does not seem to be in Postybirb. Other Electron-based browsers work fine (ie. Chrome) and it's really just a stripped-down version of that. It seems to access and interact with every other site with no problems.

I can still log in by copying my session token from Firefox, but even when logged into the website, I can't upload anything. If I try to upload an image, it gets stuck around a few % and won't budge. The upload never completes.