I think it expects a fully qualified domain name. I doubt QlikCloud knows where your SQL Server lives. SRVCB25 may work fine within the network of your organisation, but I'm pretty sure that the server can't be found with only that name from the internet, where QlikCloud lives.
I replaced the name server by the IP server but now I get this error:
Also I found the information below:
Connecting to data sources from a private network To request data from your data connections through a firewall, you need to add the DNS name for Qlik Cloud and the underlying IP addresses to your whitelist. Data connection requests are made to DNS name: qlikcloud.com.
Do you know if that information is pretty necessary?