Steps to reproduce the issue

Install 4.0.0-beta4-dev with default settings.
Login to the back end and enable the plugin System - Additional Accessibility Features.
Set 'Site Section' to 'Administrator (Back End)'.
Log out.
Wait until the session has timed out after 15 minutes.
Type in username and password, click 'Log in'.

Expected result

The page displays a message "Your session has timed out. Please reload or refresh the page to continue."

Actual result

The login request is not sent and no error message is displayed.
The javascript Console shows JSON.parse error: Uncaught SyntaxError: Unexpected token < in JSON at position 0

System information (as much as possible)

Joomla 4.0.0-beta4-dev of 4 August on Wampserver 3.2.2 and PHP 7.4.8
Google Chrome v84 - error
Microsoft Edge v84 - no error
Firefox v79 - no error

Additional comments

Javascript JSON.parse error:
Uncaught SyntaxError: Unexpected token < in JSON at position 0
at JSON.parse ()
at onSuccess (admin-login.min.js?8ab02e6f440297c5c60ef859413a6b42:1)
at XMLHttpRequest.d.onreadystatechange (core.min.js?8ab02e6f440297c5c60ef859413a6b42:1)

This error occurs only when the plugin System - Additional Accessibility Features is enabled.
A meaningful message should be displayed instead of a frozen login page.
This is an important UX consideration for those webmasters who need the Accessibility features.