>> "One possibility that does spring to mind is the old CloudFlare issue whereby we were getting some people blocked if they were coming in from "suspect" IP ranges - always possible, depending on the internet provider they are using."
That's a possibility, but only if they're routing traffic not-from browsers via a separate proxy.
In other words, if they can contact wyDay from a browser (and they're not doing any proxy shenanigans) then they can likely contact wyDay from TurboActivate.
So...
1. Use the latest version.
2. Disable firewalls. Just use the built-in firewall and anti-virus in Windows. Everything else is garbage.
>> "User then attempted an offline activation. When activating using the provided response file, gets the error message "The activation response file is invalid. Use the unmodified file provided by...". I have never seen this error before, but generated and sent a new response just in cast of any corruption. Same error, and for completeness this was also tried via our application and TA.exe."
Don't use old activation request files. Generate it new. Use it in LimeLM to get the response, and make sure its unmodified by any systems.