Thank you
I did the plug-in check using ‘Health Check and Troubleshooting’.
It seems there is a conflict between BadgeOS and Shoreditch. It didn’t occur when the Twenty Twenty theme was enabled in Troubleshooting Mode.
The plugin compatibility tool in Health Check did identify that many of the plugins we are running, including BadgeOS aren’t tested with PHP higher than 7.3 so (after doing a complete backup) I reverted to PHP 7.4, problem remained, then 7.3 problem resolved.
I’ll draw Learning Times’ attention to the issue and watch for testing of future updates of BadgeOS with PHP 8.