Steps to reproduce the issue

In Joomla 5 go to Users > Manage, click on an Enabled icon to enable or disable a user account

Expected result

The user account should be enabled or disabled

Actual result

Instead an error occurs. And when you click the back button, the enabled/disabled state has changed anyway.

An error has occurred.
0 Joomla\CMS\Event\User\AfterSaveEvent::setErrorMessage(): Argument #1 ($value) must be of type string, null given, called in /usr/local/apache2/htdocs/libraries/src/Event/AbstractEvent.php on line 205