Hello @RolandP ,
Yes, I am entering 12.11.23. See the screenshot below, which is the same as yours:

But when I hit Next, I get to the screen I posted before.
@Krist: you could be right that it’s a temporary issue at my end, but I’ve been trying all afternoon. I guess I’ll take your advice and not bother with the seat reservation, although it’s frustrating that the system won’t let me buy one.