If you're using Firefox, this is probably the same as T151770: Frequent loss of session data in Firefox (since around 2016-11-28). If you're not, it's possible your session just randomly expired, especially if you had Special:UploadWizard open for a long time (hours) before starting to upload. I've glanced at our error logging data (see T156694 about making it public) and I don't see a larger than usual number of these errors, so it was an isolated issue.
There should have been a a button shown like "Retry failed uploads", and clicking it should have successfully uploaded the file. (There were some technical-debt-related reason why it wasn't retried automatically, last time I looked – maybe we could improve this today.) Was there a button like this, and did you try clicking it?