Steps to reproduce the issue

With reference to PR #43376 (Fix TinyMCE media button, frontend edit), which will be released with 5.1.1 and fixes a problem relating to the insert media button not working correctly on the front end.

I'm using 5.1.0 and had this problem but it was solved (on a test site) with Joomla_5.1.1-rc1-Release_Candidate-Update_Package - EXCEPT that when a front-end user first clicks on the CMS media link the pop-up window contains the message "You have been logged out". This is not true, as the user can continue editing and saving articles. I am using a custom template; not Cassiopeia. The html for this message is:

   <div class="alert-wrapper">
     <div class="alert-message">You have been logged out.</div>
   </div>

Expected result

Not to see that message

Actual result

As above

System information (as much as possible)

Additional comments