This error usually appears when we migrate our Prestashop store to another server that does not have the mcrypt extension of PHP installed.

Rijndael is an encryption algorithm that makes use of said extension, so you have two options: install mcrypt on your web server (which can be complicated if it is a shared one) or disable that configuration by executing the following statement in your MySQL database:

UPDATE ps_configuration SET value=0 WHERE name="PS_CIPHER_ALGORITHM";

Remember that if you do not use the default prefix "ps_", you must put yours in "ps_configuration" becoming "yourprefix_configuration".