I am seeing an issue where saving an MD file in Safari generates an error message that the file cannot be saved (same error you show) when in fact it is. Removing http2 from Nginx config solves the problem.
In general I can upload files but am back to the situation where I cannot drag-and-drop into the file list area in the browser. I have to drop the file in the top button bar or left side bar area to upload it. This is a separate issue and has been reported earlier (Upload problems with Firefox 67.0.2 and 7.0.2 CE).
Nginx 1.14.0, Ubuntu 18.04, Seafile CE 7.0.3.