https://store.steampowered.com/app/1109209/Train_Simulator_NorddeutscheBahn_Kiel__Lbeck_Route_AddOn/ And also some others which I do not remember for the moment.
Does this happen if you restart TS and then play on the route again? I've had issues at times where the screenshot function will stop responding, but after restarting the sim, it fixes the issue.
Is it possible the Screenshot folder has changed and you are taking pictures but you are unaware of the save location?
No, never. Do you use Steam or TS to take the shots? I use F12 (Steam) and it always works. In-game I can imagine doesn't work if (1) you manage to hide F3/F4 HUD, (2) you aren't driving any train and thus don't have a HUD, such as at the beginning of a Free Roam. I can also theoretically imagine a script failure which prevents the action on the HUD. I've come across HUD failures myself, but it wasn't route specific, rather during a scenario. ps.: Is it crazy if I know the location of your profile picture precisely?