Cname redirect unstable "WSARecv() failed"

Hello!
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 no-ip.info 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 cloud.mydomain.com as cname to mydyndns.no-ip.info 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 “cloud.mydomain.com”, so I don’t understand why it is sometimes working and sometimes not.
After restoring nginx and seafile server config files to mydyndns.no-ip.info, the zip download is completely stable again…

Thanks for any hints to solve this issues!
Kind regards
Ruediger