"Private file invalid access token!" is the type of message you can see if you set files to be private and use Iframe to copy-paste code. This way you will generate a token, and that token copied will expire in 24h.
We know this is problematic but that is what a private file means and it is meant to expire in order for content to be closed off from other people if they share links.
We would suggest making the file Public and just enable Domain-level protection, that way even if someone shares a link I can't open it as it's not coming from your website.https://support.publit.io/hc/en-us/articles/360007703177-How-to-enable-disable-Domain-Level-Protection-
Other options are to use a Plugin or SDK to retrieve fresh urls (with new access tokens). https://publit.io/docs/#download-sdks
* If you have a WP website you can use WP Plugin to insert private files into the post, that way Publitio will auto-generate tokens for each visitor you have on the website.
Here is a link to the tutorial on how to do it.