Public CiviCRM forms not working for anonymous users

Question

Public CiviCRM forms (contribution pages, registration for events, mailing subscriptions, etc.) works for logged users but when we try to use for anonymous users they don't work.

For example, if we submit a registration for event the form redirects to itself without any error message.

We run CiviCRM over Drupal 7.

Answer 1

After a long frustrating debugging I found the answer on this two pages:

Installation and Configuration Troubleshooting

Anonymous Users Cannot Contribute

For Drupal sites, ensure that uid = 0 exists in your Drupal users table. This is required for anonymous access to CiviCRM pages and forms to work properly.