Joomla 3.7 introduced the custom fields as part of the core. It was a great feature missing in previous releases. However you may experience some issues while upgrading your site and trying to use it.
If you face the “illegal mix of collations issue” and already tried to fix the collation in phpmyadmin or using AdminTools extension, you can try the solution below.
This error usually appears when trying to access the Content Fields or creating a new custom field in the Content manager. You will need access to phpmyadmin in order to perform the query.
The solution itself is simple and should be a straight forward process, however, don’t forget to always backup your database and/or files to avoid any headaches, it’s always better to be safe.
Once in phpmyadmin go to the SQL section and perform this query:
ALTER TABLE #__languages MODIFY lang_code char(7) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin NOT NULL;
Replace #__ for your Joomla website prefix for database tables and execute. You should see a green confirmation message.
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