Teams preview service receives "Log in with Atlassian account" for existing Confluence pages, but successfully previews newly created pages. Can you confirm whether metadata for Confluence shortlinks (/wiki/x/...) changed recently or whether there are known issues with external preview crawlers?
Can you be more specific about your setup?
Confluence Free, which you appear to be using, does not support anonymous access. In other words, only users with a seat (Atlassian Account), or marketplace apps that you install on your Confluence site, can access the content.
There are ways how to make Confluence Free content available to the public, but you need a Marketplace app.
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