Really can't say, but I suggest you investigate the possible causes listed in the error message. Testing the network connection (a ping should do), and the possible offline service (type "service" in the windows search and open the service console and verify the Qlik services are running). For too many open connections try when the server is less busy (outside normal operating hours -- best), or disconnect everyone and try again (not to great), or in the worst case failing the above, e, restart the services or the entire server.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein