Latest version of the game as of now, Steam, Windows 10.
Basically, it says I can’t send the card because the train limit of the Grenadero is reached after revolting to Argentina as Germans, per default, without ever sending the card. Map is Unknown, but I never had this issue before.
At first I simply assumed it must have something to do with the Hungarian Grenadiers from House of Habsburg I spawned beforehand in Industrial Age using Radetzky March. My theory was basically that it somehow conflates these irregular grenade troopers.
HOWEVER, after playing another round (Alps) to reproduce it and make a picture, I could send the Grenaderos without any issues after revolting. So I don’t know what causes this. It may or may not be related to Unknown but it can’t be reproduced.