I am assuming this is QlikView as mentioned in your description and not Sense, as it says in the header?
Either way: If you lost connectivity to the QMC (https://servername:4780/qmc in your case, since you mentioned you are using https) externally, but are still able to access it directly from the server, then you are likely looking at a change in firewall or proxy settings.
Verify that all required ports are still open.
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