I can't replicate the error locally, can you update the following details at https://stackideas.com/dashboard/site so we can investigate this issue on your site?

1. your site backend login credentials
2. FTP login access
3. What Joomla version are you using now?
4. What PHP version are you using now?
5. Can you try temporarily switching your site template to the Joomla default template and see if you can see the full PHP error backtrace when you submit the registration form? Ensure that you have enabled the Joomla to debug mode from the backend > system > global configuration > system

i enabel debug mode and view backtrace
http://dl3.joxi.net/drive/2021/12/08/0002/3139/154691/91/d74349ccd7.jpg

about FTP access - i ask my colleague) may be i can give it later

joomla 4.0.4

According to the error backtrace log, it seems like the system trying to notify the site admin for the new registration email notification then caused this error.

Yes, we need your site backend and FTP login access.