Can you confirm that it is probable that the error appears if a firewall is blocking access to the update/download website? What about a proxy server?
Yes, the server file is failing to be read because either your server or intermediate proxies are serving wyUpdate garbage.
You can explicitly specify which proxies wyUpdate should use.
If so, can I overwrite the error message somehow that it states more clearly that the firewall/proxy server is blocking the update?
It depends on how your servers & proxies handle it. If the server/proxy sends garbage data then you'll get an error like you've seen. If the server/proxy just rejects the connection then you'll get a more sensible error about not being able to connect to the site.