This is a bug that stems from Seminar (see: http://seminar.vollmar.ws/index.php?option=com_kunena&Itemid=13&func=view&catid=4&id=4270). There used to be a posting with a guide to update the wrong code in the php files, but it has been deleted.

When using the old booking form, the custom input fields will be disabled X days before the closing date of an event, due to the setting: Cancellation possible till (days) in the back end. (Where X days is the number you input in the Cancellation possible till (days) field in the back end)

Work around is to set the closing date to a date after to end date of the event and then manually changing this date back to its original closing date when that date has arrived. You just have to remember to do this...

Could you look into this and correct this if at all possible?