WordPress 6.4.2 should would just fine under PHP 8.1.27.
This problem may be a plugin or theme conflict. Please attempt to deactivate all plugins and switch to the default Twenty Twenty-Three theme. If the problem goes away, re-activate them one by one to identify the source of the problem.
If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to deactivate all plugins and change the theme for you while you’re still logged in without affecting normal visitors to your site.
Many thanks for the advice. I’ve installed Health Check and gone into Troubleshooting mode – and this time Edit_User worked.
I’ve tried enabling some plugins with the bulk enabling but got the message “site failure occured” Likewise it seems whatever I try to enable a single plugin I get an error and have to go back and do “enable anyway”.
Anyway, to cut a long story short, the plugin [Simple User Profile] is the only one that causes an error and coming out of trouble-shooting mode and deactivating that has fixed the problem.
Thanks very much for the expert advice and guidance.