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The cause of the problem is that you have changed the database prefixes but still, some of the word press config is tied down to the previous prefix (wp_ or something)

Just execute the following two queries

-- Replace 'prefix_' with your actual prefix
UPDATE `prefix_usermeta` SET `meta_key` = REPLACE( `meta_key` , 'wp_', 'prefix_' );

-- Replace 'prefix_' with your actual prefix
UPDATE `prefix_options` SET `option_name` = 'prefix_user_roles' WHERE `option_name` ='wp_user_roles' AND `blog_id` =0;