When you click on a "read online" (or "mirror page") link from a received message sent via Engage, you are taken to the corresponding online version of the message.
The read online link contains specific variables that are sent to an Engage web server that contains a message ID and recipient ID, and these are used to register statistics, and the server responds with an HTTP redirection to a "one-time" link that can be used to display the page once.
You can see this difference by checking the actual read-online link in the received message where you can see that there are message-specific values sent in the query string and comparing this with the link that displays in the browser for the actual rendered page which is totally different.
Once you have received this "one-time" link, it can never be displayed again, even for the same user. This is to avoid sharing the read online link directly from a browser, and therefore creating invalid statistics.
You can generate this 408 Request Timeout error by clicking on a read-online link in an email that will open a browser to display the message correctly, then refresh this page. This will generate an error as the link cannot be re-used.
Given the redirection between the original source link and the redirected link that is used to display the one-time page, there is a difference between the link originally sent in the message and the link that is displayed in the recipient's web browser.
As this is a one-time use link generated from the parameters in the original source link if you share the final redirected link (the link containing a "sysreq" variable), or you refresh a page where a page has been successfully rendered for an extra display, this request will fail with a 408 request timeout as it cannot be re-used.
If you need to share a read-online link, you will need to share the actual source link taken from the received message (right-click on the link, copy hyperlink in MS Outlook), and not the link is taken from a render displayed in a browser.
Finally, note also that functioning read online links can only be used from full group sendouts, as these links will not be activated from a preview.
Knowledge Base Reference ID: 202202271339