Whiskydrinker

I have a webdav service running on Freenas on my internal network. Now I want to sync my iOS device using that service. So I open the freshly installed app and choose the restore data option. Then I enter the URL, username and password and click on connect. And then I instantly get an error message that the connection failed. The strange thing is that the macOS version works fine with the same server. So what is wrong here?