You can't show a message in combination with a scheduled event. A scheduled event is not a user, so Mendix will not show the message, because the message cannot be delivered to a specific client.
So you need to do it otherwise, maybe setting a boolean when your attribute is changed and based on this boolean show a dataview with a predefined text with your message. You could use conditional formatting to only show this message when the boolean is true.
Edit after answer:
Create a boolean attribute on your user entity with default value false. Something like 'Message'. When you're attribute is changed set this boolean for all users on true. Now you can create a microflow which shows a message for all the users where the boolean is true. After showing the message, you set the boolean on false for this specific user.
You can show this message each time some one logged in your application by setting a microflow as your default home page. Retrieve the boolean of the currect user, when it's true, show the message, else show your home form. You can show this message with a popup and a predefined message in another object, something like Message. You can configure this message, because it will be an entity.
We use this to show all users a message when we deploy a new release. Each time you change your attribute, the boolean will set to true for all users and they will see exactly one time the message.
The problem is that the value is changed by an user, when this happens al other user should be notified. The only way to detect this is via a schelduled event. But because a scheduled event isn't a user even't (like you pointed out) there is no way of notifing the other users. I hoped there was a way around this but it looks like it's not posible with Mendix :-(