Combobox problem

Hi,

ich watched the tutorial complex forms 3 (http://railscasts.com/
episodes/75) and included it into my application. everything works
fine so far but if i want to change

Code : ruby - fold - unfold

   1. <%= f.text_field :installed_since%>

in

Code : ruby - fold - unfold

   1. <%= f.date_select :installed_since %>

it comes to the following error message when i run the function create
or update:

"We're sorry, but something went wrong.
We've been notified about this issue and we'll take a look at it
shortly."

I can see the date_select component and change the date everything is
fine, just when i hit the button it comes to this wiered message.

can anyone help me ?

the message shows up when create or update is excecuted. eg you hit a
button update then you get to the side with the text "We're sorry, but
something went wrong.
We've been notified about this issue and we'll take a look at it
shortly."

the error appeared for the first time and only when date_select is
involved. i am just developing on my own machine and i am running the
developer mode. i could not find anything in the log yet but i am
still looking.

the question is if i am the only one where this message appears? the
problem ist date_select but what is my mistake?