New uploads failing with 400 error

[kevinsegal]

Install, migration, and url redirects all worked beautifully. But when trying to upload a new photo, it starts and then displays an error message:

DSC_0677.jpg
HTTP error.

I'm using HTTPS; all other previously uploaded content is serving successfully from S3.
It says on console it's a 400 error relating to /wp-admin/async-upload.php
I'm trying to debug myself, but if you have any suggestions I'd greatly appreciate it!

[stuartshields]

Confirmed this issue is happening for me as well:

When uploading a new image the following errors are returned in console (Firefox):

XML Parsing Error: no root element found Location: /wp-admin/async-upload.php Line Number 1, Column 1:

In Chrome we get the following error:

Failed to load resource: the server responded with a status of 504 (Gateway Time-out)

Refreshing of the page, shows the uploaded image thumbnail, sometimes this thumbnail is left blank as the thumbnail size wasn't generated by the script.