If you create a page with tasks assigned to someone, it will add entries into their "Open Notifications" task list area, and those notifications have links to allow you to open the page/tasks.

If you change the URL of your Confluence instance (which we did via export/import), old tasks have the old URL embedded in them so when you click them, they do not work.

See screenshot of a task created for me months ago, and since then we migrated to a new instance/URL.