Hello,
I am using a apache2 reverse proxy in front of a grampsweb docker. The grampsweb container is set up with the standard docker compose file. Uploading media to the container without the reverse proxy does not lead to the 413 error (there it does not work as well but that is a 2nd issue and I will open another thread for it).
I tried to find settings in the reverse proxy to change this but I did not have any success.
Just when I am writing this I had the idea to stop the adblocker uBlock Origin 1.53.0 I use in firefox (chrome did not work either but I think I have an adblocker there as well). That actually solved the issue and I can upload not only the small 40kB zip but also 2 and 7MB. I will later try my complete media archive.
Should someone else encounter this issue it might be helpful.