Nextcloud server not listed in Windows' "Trusted sites" caused timeouts

Some big files, especially compressed archives like zip or bz2, caused timeout-errors when synchronized with clients inside our safe network.
The NC server was accessed via proxy and reverse proxy with it’s official internet address.
We found out, that window’s TCP buffers were full what caused ZeroWindow errors:
17954 Client -> Proxy: TCP ZeroWindow
17955 Client -> Proxy: TCP Window Update
Although the network is faster using the server’s internal name, those errors didn’t occur when accessing the internal names w/o any proxies.
Finally we found that adding the NC-server’s external name to Windows’ “Trusted Sites”, found in those Windows 95-like internet settings thar are mostly associated with MSIE caused the erros to disappear.
Seems, windows scans or checks files although Defender and our alternative malware protection were switched off.
Same problem may occur with fast internet connections and before blaming your internet service provider you should first add the server’s name to “Trusted Sites”.