Workaround:

Adding __serialize and __unserialize functions to User make form login possible.

    /**
     * 
     * @return string
     */
    public function __serialize(): array
    {
        return ['id' => $this->id, 'username' => $this->username, 'password' => $this->password];
    }

    /**
     * 
     * @param array $data
     */    
    public function __unserialize(array $data): void
    {
        $this->id = $data['id'];
        $this->username = $data['username'];
        $this->password = $data['password'];
    }