But thank you for the hint;-) We adjusted the configuration a little bit, and at least from my iPad it is working now (iPhone is not available right now) I had some errors from the Web-Client when using external access before adjusting the nginx config - those are gone too
We added
ssl_session_timeout 10m;
proxy_read_timeout 10m;
proxy_send_timeout 10m;
#Disable size limits
client_max_body_size 0;
Not sure which one did the trick ;-)