If you're seeing this error on the first step when uploading files, it is likely due to this bug:

https://bugzilla.wikimedia.org/show_bug.cgi?id=40048

Disabling the user preference "Chunked uploads for files over 1MB in Upload Wizard" under "Experimental" in the "Upload Wizard" tab should resolve the issue. This preference is off-by-default, but some users may still have it enabled from our last round of testing, and it looks like recent backend changes broke it.

Erik