Redirects from Redirect system plugin (if enabled) cause internal server errors 500
From Joomla! Documentation
Joomla 4.0.6 (only) has a small regression reported affecting only sites that use the System - Redirect plugin. This plugin is disabled by default, so only very few sites may be affected. If your site uses this Redirect plugin, either stay at Joomla 4.0.5, waiting for Joomla 4.0.7 (which will be released only if needed) or next scheduled Joomla 4.1, due February 15th 2022. If you use this plugin and already upgraded to 4.0.6, you can apply one of the easy fixes below.
Bug report #36730: System Redirect Plugin has a regression bug in Joomla 4.0.6 affecting only its redirects which display Internal Server Error 500 instead of redirecting.
This pertains only to Joomla! version(s): 4.0.6
What is the cause
This is due to a PHP error in the Redirect plugin on updating the corresponding redirect counter, that is uncatched.
How to fix
If you use the "System - Redirects" plugin and already upgraded to Joomla 4.0.6, you have the choice of one of these fixes:
- Once Joomla 4.1 (or 4.0.7) are released, upgrade.
- either downgrade your site to Joomla 4.0.5,
- or temporarily disable the "System - Redirect" plugin
- or apply the bugfix #36733 by editing file plugins/system/redirect/redirect.php and changing the part "$db->" to "$this->db->" on line 243 as shown in the diff on bugfix #36733.