After upgrading a Magento 2.1 instance to Magento 2.4, I realized some problems with serialized data. In Magento 2.1 PHPs serialize() and unserialize() functions have been used whilst in Magento 2.2 the default serialization uses json_encode() and json_decode().
Recently, we took over a Magento 2 project where data was migrated from an older Magento 1 instance. The merchant complained that after the migration some customers are not able to log in anymore. Even a password reset did not work.