Microsoft Internet Explorer allows a default maximum of six concurrent WebSocket connections to a single server domain. Each virtual machine console window, or serial console, uses two WebSocket connections. As a result, the maximum number of virtual machine console windows that you can open in Internet Explorer is three.
Workaround: Do one of the following:
Refer to the appropriate Microsoft documentation for information on changing the maximum number of concurrent WebSocket connections.
Use a different browser, such as Mozilla Firefox.
Bug 21617374