I see "isClient" or "isAdmin" errors after upgrading from Joomla 3
An old extension is still active. Disable it (in the database if the admin won't load), then update or replace it.
Why it happens
Joomla 4 removed old methods such as isAdmin() and isSite(). An extension that still calls them stops with a fatal error, and it can take the whole administrator down with it.
Fix it
- Turn on error reporting (set
$error_reporting = 'maximum'in configuration.php if the administrator doesn't load) and read which file the error comes from: the folder name tells you the extension. - Disable that extension: in System → Manage → Extensions, or in phpMyAdmin by setting
enabledto0for it in the#__extensionstable. - Install a Joomla 4+ version of the extension, or uninstall it and use a replacement.
- Turn error reporting back to System Default.
Good to know
- Check all third-party plugins: a system plugin from Joomla 3 is the usual suspect because it runs on every page.
Sources
- Joomla 3.10 to 4.x Common Migration Errors – Joomla Documentation
- FatalError – Joomla User Guide
