"flight number assigned"-error, but route created

Same for me, on desktop / Firefox (and Chrome). Very consistent behaviour. In my case, as follows:

1. Go to Manage Routes.
2. Click the "+" sign to create another route like this one.
3. Change date by one, let's say next day. Click Save.
4. On the resulting green screen, click Create a route for Next Day.
5. In the new route screen, do nothing, just hit Enter to save.
6. In the next screen, the route gets created BUT a red error box pops up and the same day is selected.

(so therefore it is impossible to create more than +1 day's worth of "next day" routes).

Adding iPad with safari browser

Same for iPad… my steps slightly different but same result. In my case I built a 7 day schedule. I go to scheduling page and hover on route and select copy this route
Change day to next day (I.e. Monday to Tuesday)
Change flight number to the one I want to use [i.e. AW001 vs new default # AW100)
Hit confirm
Get blinking error message that route already exists
(However, route IS created)
So go to new plane and schedule the route to it.