I really wanted to try the game, but i cannot find the manual.

I tried to start a Campaign pressing Campaign in the main menu.
But all that happens is, I get a textbox for some text, no description what the text is good for.
No button either to continue or want ever.

What am i supposed to do there?

I can't remember if there were something else when starting the game for the first time, but by pressing Campaign you should be getting an option to choose your save file. There are three different boxes that each tells you to click the letter icon to begin, which then gives you the option to name your save file before starting your game.

Do you get something different?

I get the three textboxes, but there is no description on them what they are good for, good to know that these are the save files names.
But how do I start now? There is no button or anything else clickable on the screen.

It should be that letter icon on the left side of those save file boxes (attached pic to clarify), which when clicked lets you rename your save and start the game.

Don't you have anything there?

If that's the case, let me know and I'll try to look into it more.

EDIT: I looked it up just in case. If you can open the options menu when you start the game by pressing Esc at the menu, from there go to the Options -> Graphics -> make sure Fullscreen option is not checked. It seems like some people has had similiar issues and running the game in Windowed rather than Fullscreen has helped them.

Hopefully it does help you too, if not, let me know and I'll see if I can find something else.

My screen looks exactly as yours, no description and no buttons beside the 3 letters and the options candle.
I can enter text, but i have no idea how to start the game.

Ok pressing enter did the job, now i'm stuck at a screen with title Hamlet and no interaction possibility again.
I think i should just delete that game, the interface is really bad.

Are you in this screenright after Intro movie (pic)? If it's this one you can only interact with it after it's loaded enough. After that it throws you to the tutorial section. If you can't see the text marked in to the picture, then try playing Windowed mode as it may crop out the picture for one reason or another. At least to try out if that's the reason it might not show up at all no matter how long you'll wait.

If you already got past that, you'll get to the town itself, where you can only interact with maybe a wagon at most, and then you'll have to go to the first dungeon. Those will periodically open after played through the first few dungeons.

It was worth to try at least, hopefully finding a better experience with the next title you'll decide to play.
Happy gaming!