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Did the download finish after you had deleted the folder with its content? If so, then everything should be fine! And in that case you don't need to delete the destination folder anymore. The installer + game installation are both targeted to that folder.

Also this game into my mind..   do you have enough free disk space available? The installer requires around 3.8 GB, and the installed game takes about the same amount.

Thank you for reporting the issue with the game. 

I would suggest navigating to your %LOCALAPPDATA% folder and looking for a folder named HoroxAlptraum. 

Please delete it (=   HoroxAlptraum)   along with its contents and then try the Launcher again. 
If it’s only related to some metadata or something similar, you could try reinitializing it first. 

Let me know if that works? :)