And after stumbling blindly around I finally found the Firewall logs, switched the filter to just show block events, and indeed found blocks coinciding with each failed attempt to Connect with the Roku for Miracast screen mirroring. Took me far too long and too much looking in the wrong places (thinking it might be the W10 1809-1903 update) before finally remembering last time, when it worked, I was using Comodo Internet Security, whereas now I'm using Avast Premier. Last time I used screen mirroring from the laptop to the Roku a few months ago (via the Connect button in the Action Centre) it worked fine now it isn't working - the connection starts to connect, then stops before the laptop screen would appear on the TV, and I get a failure to connect message from the laptop. I have a windows 10 laptop (1903) and also a Roku Express connected to my TV: almost all the time I use the Roku separately from the laptop for NOW TV and Amazon Prime on the TV, but very rarely I want to use the Miracast screen mirroring to put my laptop display on the TV via the Roku (the TV itself does DLNA, so I can use it as a media renderer for media servers on the laptop, but not directly to mirror the screen). Was a time I was good and knowledgable about internet stuff - disability and brain-fogging medications mean I now can't remember most of what I used to know or think clearly, so while I think I have a specific question, I'm going to start with the problem and what I think I've slowly figured out in case I'm actually onto the wrong thing and somebody can see that.

Lonely screen blocked by firewall