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Hello,

Can you active the Joomla Debug to maximum level in your Joomla Global Configuration, System tab and server tab :

And provide a screenshot with new error elements, but so far there are good chance that this isn't linked to HikaShop, but directly with Joomla 4 compatibility.
The best way to see this is that even on Joomla Registration form we are able to reproduce your described error ( here ).

This error message come from Joomla and it displayed when "something" redirect with an error message, the plugin that create this error should not actually be able to handle Joomla 4.

The best way to get the error root would be to unpublished system and user plugins one by one and to try to reproduce until the error no longer appears in order to get the plugin that create this error and his maybe not up to date to work with Joomla 4.

Regards

Hi,

These are only the plugins of the group "system" with the text "user" in their name.
You need to try deactivating each plugin of the group "system" OR of the type "user". You should have a couple dozen of them, not 3.