This happens regularly to me with an MSC using Sassie. The calendar dates are grayed out. I can type the date into the box (when will you complete this shop - or some such), but without highlighting the date on the calendar, it's a no go. I then have to email the scheduler, which is a pain, as I should be able to self-assign. The scheduler advises that it happens to her also, and she has to "work around".