I had the same issue and found it to be related to Seafile’s NGINX combined with my own NGINX. It doesn’t like the double proxy.
My solution was to edit Seafile’s NGINX conf with the required settings for SeaDoc and notification-server, and keep my own NGINX conf very simple. Here’s how I did that.
Hopefully the devs can improve this so that users with their own NGINX don’t keep running into the issue. I really don’t like how Seafile works under Docker. Docker containers are supposed to run single processes, but Seafile’s containers are hacky and run multiple processes, as if trying to emulate a virtual machine setup. This is just wrong. Also using NGINX within the containers is just wrong - they should have a separate NGINX container, which we could optionally replace with our own NGINX if we needed.