Ok thanks for clarifying. In my case I am the maintainer of the Nextcloud instance and I definitely don't trust myself in that regard 
I remember having these timeouts with Dropbox when I was on a very slow ADSL connection so it's not limited to Nextcloud.
As I said, a network timeout should only occur, if there's no data on the wire. e.g. you try to connect to a server, but there's no reply for X seconds. Then this is called a timeout.
If we limit a network session to a certain amount of time, no matter whether there's a data transfer in progress, we should change the error message and the code. This is not a timeout.
This is the solution. I deleted two attched large files in -resources And than the sync started again an now everything is synced. After that I started the sync on iphone and now it works.
Thanks all a lot for the support
How big were the attachments?
They were both videos with 150 MB