Cannot create user following upgrade to

The OJS has recently been upgraded from 2.4.8.
When attempting to add or modify a user I see: “Failed Ajax request or invalid JSON returned.”
accompanied with the following php log:

PHP Notice: Only variables should be assigned by reference in /var/www/html/lib/pkp/controllers/grid/settings/user/form/ on line 176

As far as I know the upgrade was successful:
bash-5.0# php tools/upgrade.php check
Code version:
Database version:
Latest version:
Your system is up-to-date

A PHP Notice should not be enough to cause the AJAX failure, unless you have display_errors turned on in If display_errors is turned on, turn it off for production purposes.

If display_errors is already off, look for another message in your PHP error log, probably with a “PHP Error” or “PHP Fatal” prefix.

If there are are no interesting errors in the PHP error log, and if you are comfortable using the browser’s web inspector, capture the result of the AJAX call and share it here.