Hi Mats,
I did some more looking into this and found how to produce the problem. If I just click the main link to add an event it works correctly, but if I start on day view and click an empty time slot to add an event it results in the checkbox is not checked and the dropdowns are disabled. As you say clicking the checkbox and unclicking it again enables the dropdown lists so it is a minor bug, but I will fix it now so it will be fixed in the next update of Event Calendar Pro.
Thanks,
Joe