Sorry it's taken us so long to get back to you on this! I've gone ahead and tweaked the error message to something more human-readable. There are multiple ways this error can be triggered, so I can not actually have it say "This event has ended" but the new message will be at least less jarring.

You could post this in the suggestion forums, but I can tell you this won't be changed... It's the same reason you can't, for example, accept a dance immediately after Masquerade closes. The event is either open or not, so there's no gray area where the event is closed but you can still do the event things.