Notice: unserialize() [function.unserialize]: Error at offset 0 of 13 bytes i UserController->attachLoad()? I've created my first module, which gives me this error and I dont know where to look for the error.

The module is a list of nodes sorted by terms. Each node has a checkbox connected to a form (using Form API), which lets the user sent multiple emails at once to the author of that node. The data is then inserted into the database for further use. The funcion seems to work and the node authors gets its own rows in the table, but why this error?

It is shown when the form has been submitted, btw.

Drupal 7

Notice: unserialize() [function.unserialize]: Error at offset 0 of 13 bytes i UserController-&gt;attachLoad() (row 303 of /drupal/modules/user/user.module).
Take a look at the Variable Check module (http://drupal.org/project/variablecheck). That will give you a list of the variables stored in your db that are causing the problem. Once you've identified those bad variables, you can delete them from the 'variable' table in your database.