If I try to upload the file using the old Special:Upload page, I get the error message "413 Request Entity Too Large / nginx/1.6.2".

I think I have discovered what caused the error. I decided to retry the upload today, and noticed that yesterday when I had attempted the uploads, I had forgotten to complete the required fields "Source" and "Author" on the page before the "Describe" page. However, despite not having entered any information into those fields, I was able to progress to the "Describe" page, though not to click the "Next" button. There is also no way to go back one step once you get to the "Describe" page.

Today, when I filled in those fields, I was able to complete the upload (https://commons.wikimedia.org/wiki/File:Montesquieu,_De_l%27Esprit_des_loix_%281st_ed,_1748,_vol_2%29.pdf). It looks like the website needs to stop editors from advancing to the "Describe" page if they forget to fill in the fields on the previous page.

I tried to reproduce this problem, but I couldn't find a way to progress from "Release rights" to "Describe" without choosing one of the three options and filling in all the fields required for the given one (the "Next" button is not visible, or I get error messages). This must have been fixed since April.

I'm going to close this task, if you run into this issue again, please reopen it!

Yes, I haven't encountered the issue for a while, so I guess it has been fixed.