Tried to update last night on a Windows 10 Laptop. When I clicked on the downloaded file I got a popup that said CCleaner was not an approved program and I should shop at the Microsoft store. Would not go further.
That sounds like you have your Windows settings to only allow you to install apps from the Microsoft Store.
(They say it's 'to protect your device', but TBH it's mainly a way for them to make money).
Go to Settings>Apps and change it to allow installing apps from anywhere.
You were right on: Somehow the Micro$oft only setting was on. Nuked it last night and all is well. Thanks