Days ago, when I temporarily substituted a bigger aircraft for a smaller one, I got an oversupply notification. I fixed it as soon as I could.
I am still getting this notification:
Game notifications
Route warning
Some of your routes are flying in violation of the game rules (seat oversupply). Please see the message inbox for details.
ยป Messages
There once was a message, which went away when the oversupply was taken care of. There is nothing relevant in "messages" anymore.
When will this notification go away?
Did you check the route also from the return leg?
If no problems, it will disappear within a game week.
I finally found it. The aircraft in question was in C Check.