If you create a new Markdown file with Safari 11.1.2, it cannot be opened or edited using Seafile-Server 7.0.3 CE. Also logging out and back in does not help. Restarting the server helps. The creation and opening of a txt file works without any problems.
Just use working browser engine, not that stolen and shitty patched KHTML.
Version 7.0.4 CE fixed it. Thanks to the developer!