I happened to come across your post and saw that it was unanswered so I wanted to provide an answer. IE has a limit to the number of websockets that can be opened with a host at a single time. The default is set to 6, but it is possible that a policy is set on your machine that limits it to 4. I came across a post on MSDN that shows how you can change the number of websockets: https://msdn.microsoft.com/library/ee330736(v=vs.85).aspx#websocket_maxconn
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There is a limit per browser on number of parallell connection toward same hostname. So this is browser configuration that stops you from connecting towards Qlik Sense services. This is a browser limitation that is known to Qlik.
Opening to many tabs against Qlik Sense Hub then try to use QMC can also cause this problem.
The issue happens if you have too many connection alive against the same hostname, which is in this case as there are websocket connections opened up.