Cname redirect unstable "WSARecv() failed"

I am running Seafile Server on Windows 10 without any issues for almost two years, currently running version 6.0.7.
Until now, I used to obtain a dynamic DNS for my dynamic IP at home.
Everything worked well.
Recently, I moved my private domain to a new hoster which allows me to use cname for subdomains of my own domain.

So I have setup as cname to in the interface of my webhoster.
Everything seemed to work ok, but especially ZIP downloads of shared folders often fail.
I checked the error logs of nginx and found a lot of “WSARecv() failed” errors.
Any ideas what is causing this?
I’ve changed the nginx and the seafile server config files to “”, so I don’t understand why it is sometimes working and sometimes not.
After restoring nginx and seafile server config files to, the zip download is completely stable again…

Thanks for any hints to solve this issues!
Kind regards