I've been caught with this problem several times despite efforts to try to watch out for it...
When creating new routes, usually in sets (of 2 to 5) for a newly arriving aircraft, I have several Open Route screens open (in separate tabs). After adjusting the times in each to achieve a workable/optimal fit between them, I click on confirm on each in sequence.
Occasionally (if I don't double check again on curfews) I may have already paid for one route and find that I cannot buy the other because the arrival time is when the airport is closed. (Could it happen if slots are sold out at arrival time?)
The Open Route page nicely gives warnings when the departure time slots are not available (red zeros on the days and "No slots" in the little box). However, arrival times given no indication that there is a problem.
I suggest that the Arrival Time get checked and, if it is within the curfew period, give a red and bold highlight similar to that used for departure times.
I would guess that this is a problem using the Edit Route screen too (probably identical logic behind the scenes), though I've not run into this issue there.