Are you using a VPN of any kind?
Are you blocking any/all inbound ports on the router?
If you have any anti-virus on the computer try disabling it including the Firewall.
No VPN. I’m not knowingly blocking any ports on the router.
I am running with Windows own Antivirus and firewall (no MacAfee / Norton etc nonsense)
Cycle the power on the router.
What DNS servers are you using on your PC?
I’ve power cycled the router.
C:\Users\kat>nslookup zwift.com
Server: cache1.service.virginmedia.net
Address: 194.168.4.100
I am looking for the DNS servers on your PC.
Open a command prompt and type in ipconfig/all
I still think something is blocking ports.
C:\Users\kat>ipconfig/all
[lots of stuff]
DNS Servers . . . . . . . . . . . : 194.168.4.100
194.168.8.100
Looking at the resource monitor, would it be expected that ZwiftApp.exe (when stuck on the pairing screen and minimised so that I can type this reply) have a memory ‘commit’ value of 7,418,780 KB? ~That seems very large to me but I’m not sure what the ‘commit’ part would be expected to be.
I’ve just signed into to my router to check settings. The router (Virgin Media SuperHub 3.0) has 3 options in the advanced setting relating to ports: IP and Port Filtering; Port Forwarding; Port Triggering. None of these have anything set inside them so I assume that I am on the default settings - just as I would have been a couple of days ago when Zwift worked. So I don’t think it an odd port setting in my router.
Also - thank you for your help Paul!
Those are local DNS servers, can you check your router to see what DNS servers are listed there.
You could try changing your DNS servers to Google’s (8.8.8.8 and 8.8.4.4)
I am not sure what you mean by local DNS server - I am using the DNS server of my ISP. Unless you think for some reason Virgin Media, my ISP, has changed something in regards to Zwift itself? But doesn’t the Zwift ‘game’ actually run through AWS? In which case wouldn’t all access to anything running on AWS be compromised if it was a DNS issue? (In case you can’t tell I don’t know a lot about networking! 
)
From my Router:
WAN IP settings
Your current Hub 3.0 internet settings are displayed below
MAC address : ###[redacted]###
|IPv4 address|: ###[redacted]###
|Default gateway|: ###[redacted]###
|IPv4 lease time|: 6 days 23h:20m:26s|
|IPv4 lease expire|: 01/10/2019 18:41:02.00|
|IPv4 DNS servers|: 194.168.8.100|
The DNS addresses you posted are more than likely your routers LAN/WLAN addresses, they are non-routeable and not public IP address/Public DNS servers.
I am guessing you don’t have access to the router to change DNS settings.
On your local PC:
Go to the Control Panel on your PC (change the view to either Large or Small icons) and open Network and Sharing Center, on the left-hand side click on Change adapter setting, right click on your Ethernet connection, click Properties, click on Internet Protocol Version 4 (TCP/IPv4), click Properties , on the bottom click on Use the following DNS server addresses, in Preferred DNS server: 8.8.8.8, in Alternate DNS server: 8.8.4.4 , click OK, Close and close all open windows.
Open command prompt and type in: ipconfig/flushdns
Don’t worry this wont hurt anything and can easily be undone.
FYI, I have been a Network Admin for about 15yrs.
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Another data point to add to the above while you were typing your reply.
I just downloaded the Zwift Android Beta app on my phone. My phone is connected to the WiFi of the same router than my computer is cable connected to. Zwift on my phone logged me in fine, paired to my trainer fine, put me in a Zwift world fine and my avatar responded to my turning the pedals. I could also see other people on the course (I wasn’t alone like you get if there are connection issues sometimes). So at least I guess whatever the router is doing is not the problem?
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Using the same DNS servers?
Well sometimes devices with cell service will use both WiFi and Cell data to get were it needs to, so it could still be something on your router.
Can you turn cell service off on your Android, but leave WiFi on and see if Zwift loads.
Just trying to troubleshoot.
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ok…
C:\Users\kat>ipconfig/all
DNS Servers . . . . . . . . . . . : 8.8.8.8
8.8.4.4
So my computer is now reporting that it’s using Google’s DNS.
I turned mobile data off from my phone and left it on WiFi. Zwift started normally, behaved normally, I could pedal, it responded with my avatar moving and I could see other riders on screen.
I tried the Zwift application on my computer. It’s behaviour hasn’t changed. It still spends most of it’s time ‘not responding’ and getting ‘hung’ on the pairing screen as if it is not getting any resources from the computer to work properly - i.e. it looks a bit like it updates 1 frame every 30 seconds or so rather than 60fps.
Now I’m properly stumped!
And to make sure that it’s clear - thanks for your help and time on this Paul - it’s most appreciated!
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Ok, we have eliminated routing and DNS I believe.
Now you might not like this next suggestion, but it could be the only thing left.
Clean re-install on Win10.
I am guessing you are sure that ALL the drivers are up-to-date on the PC?
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Hang in there @Kat and @Paul_Allen. You are making progress, the more things you can eliminate the closer to a solution you will get.
Do you use windowed or full screen mode?
Do you have Zwift on your primary monitor or a TV/second monitor.
I assumed you check hard disc space.
Are your trainer Bluetooth or Ant+?
Do you have a bluetooth dongle plugged in?
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Man, I forgot to recommend Windowed Mode for Zwift, good catch @Gerrie_Delport
His disk space is good, he posted his specs a little while ago.
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Yeah - I run in windowed mode. I’m use ANT+ (I just saw the email from Zwift regarding Bluetooth issues with Win 10).
Well - it’s certainly obvious that I wouldn’t be rushing to do a new Windows install…
I am trying to think what, if anything, could have changed on my system - 6pm Sunday I finished a Zwift ride normally. 7pm Monday Zwift would no longer work. Windows update tells me that I am on the ‘Bluetooth buggy 1903’ version but I’ve been running that since the 10th September (and I connect via ANT+). And I haven’t had any non Windows Defender updates since the 11th so Windows doesn’t seem to have updated silently in the background Sunday evening without me noticing and messed everything up.
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One more Thing, are you running the Windows developers version/build (I think that is what it is called). I remember some issues with that.
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No - I am not running a developers version of Windows. Just a standard consumer version up to date with Windows Update
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I also was able to run on Android when my PC would not work — this is a Zwift bug … I do not have the tech expertise of you Kat and Paul, but nothing has changed on my machine from when it worked to not worked aside from updating my Windows after it did not work and it still did not work …