Steps to reproduce the issue

Go to User Menu > Accessibility Settings
Try to change the present configurations and select either Save or Save & Close
Without making any changes as well, same output will be generated.

Expected result

Configurations should get applied. If no changes are made, depending on the selection, same menu or dashboard should open.

Actual result

Following error message shows up Cannot access offset of type string on string

System information (as much as possible)

PHP-8
joomla 4.0.0-beta7-dev

Additional comments