Symfony 2 CSRF issue

Question

I'm having a trouble with a legacy Symfony application:

• The application has been deployed 2 years ago.
• The application was working perfectly until two days ago. Now, every user that try log in get CSRF error (CSRF invalid token)
• No modification has been made, neither software, nor on the server, NOTHING.

It's a critical application and I need to solve it. I have tried:

• Looking in Apache logs. I don't found any relevant information.
• Disable CSRF validation. That had no effect (the error is the same).
• Increase php limits (trying this answer)

Any ideas?

Some facts and versions:

"require": {
        "php": ">=5.4.3",
        "symfony/symfony": "~2.4",
        "doctrine/orm": "~2.2,>=2.2.3",
        "doctrine/doctrine-bundle": "~1.2",
        "twig/extensions": "~1.0",
        "symfony/assetic-bundle": "~2.3",
        "symfony/swiftmailer-bundle": "~2.3",
        "symfony/monolog-bundle": "~2.4",
        "sensio/distribution-bundle": "~2.3",
        "sensio/framework-extra-bundle": "~3.0",
        "sensio/generator-bundle": "~2.3",
        "incenteev/composer-parameter-handler": "~2.0",
        "friendsofsymfony/user-bundle": "~2.0@dev",
        "mopa/bootstrap-bundle": "v3.0.0-beta3",
        "twbs/bootstrap": "v3.0.0",
        "knplabs/knp-paginator-bundle": "dev-master",
        "knplabs/knp-menu-bundle": "dev-master",
        "knplabs/knp-menu": "2.0.*@dev",
        "craue/formflow-bundle": "dev-master",
        "leafo/lessphp": "0.4.0",
        "genemu/form-bundle": "2.2.*",
        "lrotherfield/form": "1.1.x-dev",
        "stof/doctrine-extensions-bundle": "~1.1@dev",
        "liip/imagine-bundle": "dev-master",
        "dizda/cloud-backup-bundle": "dev-master",
        "antimattr/google-bundle": "dev-master",
        "friendsofsymfony/jsrouting-bundle": "@stable",
        "pugx/multi-user-bundle": "3.0.*@dev",
        "liuggio/excelbundle": "~2.0",
        "fresh/doctrine-enum-bundle": "v2.5",
        "igorw/file-serve-bundle": "~1.0",
        "knplabs/knp-snappy-bundle": "dev-master",
        "hakre/dropbox-uploader": "dev-master",
        "dropbox/dropbox-sdk": "1.1.*"
    },