When I finish mission #19, I get a text saying that the next mission is a very urgent one and it has been given to another officer of Lieutenant rank or something like that.
It then plays the extro video and the game ends. Wtf?

I'm a Lieutenant... I've played the game before (back when it was new), I don't think I played much different this time and yet this never happened to me before.
Anyway, what a retarded mechanism is this anyway to demand a certain rank for the next mission and never even tell you about it? Beating the missions should be enough, imho.

Anyway, I guess I can just get the level access code somewhere, so it isn't that important.
Thanks for your reply!

I probably played it last time in the previous century, but don't Commandos have 19 missions only?

No, I'm pretty sure it has twenty, you just can't access it unless you are a certain rank. The game's justification for it is the fact that the last mission is incredibly difficult, and only someone who has truly proven themselves can access the mission. Or something like that. I myself have never accessed the final mission, but I read in several places about the rank thing.

They are indeed correct that you need to be a certain rank to get to play the last mission, which is extremely stupid, in my opinion.

The code for the last mission for GOG's version is Q8U4V