Symfony 2 CSRF issue - symfony

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.*"
},