Interesting! Do you have an nginx service in your stack? If so, can you confirm which nginx is throwing the error, your stack's, or the System-provided nginx (ie the reverse proxy)? You will hopefully be able to tell by looking at their respective log files. Let me know if you need help finding them.
Are you overriding the nginx configurations at all?
In beta 15, we modified the nginx reverse proxy configuration to include the following:
which tells Nginx to allow an unlimited body size. In rc3 we added the same to the nginx site config (used for each nginx site you define).
So if you've not overridden any nginx config, I'd suggest you try rebuilding your stack(s) and see if you get the new default config cloned in.