Login not working in Silex with PostgreSQL

I'm developing a project with a small client area with the Silex framework. I want to store the session in database using the SessionServiceProvider and PdoSessionHandler but when I try to login with a test account the session is written in the database but the login is not done correctly and I get the login page in a loop. Also I'm getting the following error in the error log

Uncaught exception 'PDOException' with message 'SQLSTATE[22021]: Character not in repertoire: 7 ERROR:  invalid byte sequence for encoding "UTF8"

The sessions table looks like this (got it from the Silex documentation page):

CREATE TABLE sessions (
    sess_id VARCHAR(255) NOT NULL,
    sess_value TEXT NOT NULL,
    sess_time INTEGER NOT NULL,
    PRIMARY KEY(sess_id)
);

Anyone can help?