Identify the Source of problem, general tip

Since you can't access the page, you need to find out which extension is causing the crash.

1. Enable Debug Mode: If you can access the backend, go to Global Configuration > System > Debug System. Set it to Yes.
2. Check Error Reporting: Set Error Reporting to Maximum in the Server tab.

Geoff, I cannot login "Username and password do not match or you do not have an account yet."

Remember that not all Templates are J6 ready, and some of them request using "legacy" mode setting in plugin.

Updated details

I am using the latest version of helix ultimate and the edulif template

Thanks for access.

1. You forgot to enable plugin "Behaviour - Backward Compatibility 6" - yes, I enabled already :)
2. Please switch to PHP 8.4 , by now Helix do not fully support PHP 8.5, but it will be in next weeks.

That's all. Solved.