I fixed it! I’ll document what happened for anyone who has a similar issue.
So what happened was that, somehow, my world’s thumbnail got deleted from the VRC servers, causing the SDK to throw the above-listed error when attempting to access it. I had this issue when trying to upload the default VRChat SDK scene in the same project, and also in a new project entirely.
The fix I found was using VRCX to update the world thumbnail from outside Unity (I couldn’t find a way to do so from the VRChat website), and that allowed the SDK to load my world ID without any issue.
I hope this info helps someone and that I’m not breaking any rules with the fix I found! 
If there was a more straightforward way to fix the thumbnail, please let me know!
Thank you folks for your help!