Here is what makes everything work ok:

In the cloud.conf (Seafile) I deleted the "http2" in the server part that listens on port 80 and redirects.

It looks like this now:

server {

listen 80;
listen [::]:80;
server_name cloud.mydomain.com;

rewrite ^ https://$http_host$request_uri? permanent; # force redirect http to https

# Enables or disables emitting nginx version on error pages and in the "Server" response header field.
server_tokens off;

}

Noe everything works fine.

I am not sure what advantage / disadvantage http2 had, to be honest.

Maybe the http2 part should only be inside the config part that configures the 443 access?

Well, this did the trick at least.

I am still interested in the config that you posted on gist though.
It looks really tidy and well organised.

So I would still like to know if I can leave Nginx running for letsencrypt bot to work ;-))

Greetings, Chris