Use of _POST vars in selection list

I have created a webform using which people can apply for a trip. One of the fields is a selection list with dates. People select which date they would like to go. The dates for each trip vary over time.

There is one webform per trip.

Let's assume we have trip T with dates d1, d2 and d3. These dates are available on the page 'before' the webform. I.e., on that page there is a button which leads them to the webform.

I did the following.
On the page before the webform I created a form with 3 hidden values for the dates d1, d2 and d3; I created a submission button which leads to the webform. This assures the POST values d1, d2 and d3 to be present when going to the webform.

On the webform I defined a select list with 3 values: %post[d1], %post[d2] and %post[d3].

When the submit button is pressed the visitor goes to the webform and sees a select list with the correct 3 dates. The visitor selects one of the dates and clicks on Submit. Now the error "An illegal choice has been detected. Please contact the site administrator" pops up. This is strange because the value should be correct.

I search the internet, the issues in this queue and still I do not know what the cause is, how to resolve this problem.

A second problem is that if the above would work correctly, when the visitor forgets to fill in another field, the webform has forgotten the post variables and the selection list remains empty.

I am stuck here. I really hope someone can help me out here. If I should use another way to pass the dates over to the webform please let me know.

Anyway, thanks a lot for this great module!!