Ok. I found the Problem.
In short; Enpass is not able to communicate with TLS1.3 Webdav shares.
I thought that already, and I changed the virtual host in my nginx configuration to TLS1.2. After that, the webdav share was still not able to connect.
Now we come to the part that I don't know yet:
I had two configuration files in my nginx reverse Proxy:
[root@server ~]# ls -1 /etc/nginx/conf.d/
01-website.conf
02-nextcloud.conf
In the 01-website.conf file, I set the TLS setting to tls1.3:
ssl_protocols TLSv1.3;
In the 02-nextcloud.conf file, which is responsible for my nextcloud instance, I set version 1.3 and 1.2:
ssl_protocols TLSv1.3 TLSv1.2;
Now I thought, that the virtual server for nextcloud is able to communicate with TLSv1.3 and TLSv1.2. But this is unfortunately wrong.
Nginx takes the first configuration file, in my case 01-website.conf, and enabled only TLSv1.3. All other methods are disabled now for all virtual hosts.
I thought, that the server name indication ensured, that nginx will offer the settings (TLS1.2) from my 02-nextcloud.conf. But I think, that the server name indication will take effect after the first TLS connection. So the only available TLS version is TLSv1.3.
What I did:
I created a new file 00-defaults.conf and enabled TLSv1.2 and TLS1.3.
In the 01-website.conf I enabled only TLSv1.3
In the 02-nextcloud.conf I enabled TLSv1.2 and TLS1.3.
Now I was able to connect via TLSv1.2 against the nextcloud virtual host and the webdav sync worked.
Cheers, Etroska