I had this same problem after using Autoupdate (1-Click Upgrade), from version 1.7.6.4 to 1.7.6.8
It all went well, the frontoffice is fine, but I couldn't login in the backoffice. Always returning to the same Login screen.
After desperate hours digging and searching here, I finally found out that since Prestashop 1.7.6.6 there are 2 new tables in the database: ps_employee_session and ps_customer_session. The information can be found in github: https://github.com/PrestaShop/PrestaShop/releases
These tables are crucial to login. Without them, we can never success the login. For some reason the autoupdate procedure did not add these tables.
Solution:
If you can access the MySQL database, add these 2 tables
CREATE TABLE `PREFIX_employee_session` (
`id_employee_session` int(11) unsigned NOT NULL auto_increment,
`id_employee` int(10) unsigned DEFAULT NULL,
`token` varchar(40) DEFAULT NULL,
PRIMARY KEY `id_employee_session` (`id_employee_session`)
) ENGINE=ENGINE_TYPE DEFAULT CHARSET=utf8 COLLATION;
CREATE TABLE `PREFIX_customer_session` (
`id_customer_session` int(11) unsigned NOT NULL auto_increment,
`id_customer` int(10) unsigned DEFAULT NULL,
`token` varchar(40) DEFAULT NULL,
PRIMARY KEY `id_customer_session` (`id_customer_session`)
) ENGINE=ENGINE_TYPE DEFAULT CHARSET=utf8 COLLATION;
Remember to replace PREFIX_ by your tables prefix (mine is the usual ps_), and COLLATION by the one used in your tables, or simply remove it from the code.
After that, I could access again the backoffice.
Note: A fresh Prestashop installation will add the tables, which is also a solution.
Nelson / Contacto Visual