If you get below error i.e. URL No Longer Exists while accessing Visualforce page e.g. after you have added conversation view to the page layout. One of the scenarios, when this error occurs, is when Clickjack Protection for Visualforce page is enabled for your Org.
You can check the same by going to Setup-> Security Control -> Session settings -> Clickjack Protection -> check if clickjack protection for customer Visualforce pages with standard headers and clickjack protection for customer Visualforce pages with headers disabled is enabled.
If clickjack protection is enabled, applyHtmlTag attribute should be added and set to false in the respective Visualforce page.
To edit the Visualforce page go to Setup-> search for Visualforce Pages-> open the Visualforce page which is throwing the error ->click on Edit and add applyHtmlTag ="false" in the first line. Please refer below image for the same.
You can refer below link for more details: