Similar but slightly different: After we upgraded from 9.0Rx to 9.1R7 we noticed employees reporting 'addEventListener' issues inside the (older) Network Connect client on Windows 10 devices. To ensure continuity we fixed it as follows, go to: Configuration -> Security -> Advanced which should bring up the "Custom HTTP Headers" page. Insert the following: HTTP Header: X-UA-Compatible Header Directive: IE=Edge   The error will then go away (as Internet Explorer is the issue here). We still have to deal with some other fallout but hopefully this will help someone else.   Cheers. (I'm adding it here because this is the only hit when searching for "addEventListener") ... View more