SOLVED: Error uploading files - FILE_SERVER_ROOT set correctly

Seafile server 6.0.6
Centos 7.2
Nginx 1.10.2
Letsencrypt ssl certs

Upload via safari or chrome doesn’t work.
Webdav seems to work.
Seafile iOS client does not work (connects to server but can’t upload / download)

When i try to upload via safari or chrome i get an “unknown error”.

How can I debug this with Chrome so I can get some hints what is happening?
What should I look at?

It would be great to get some advise.

Chris