Yes of course this is possible but why works my system without installed AddOn Manager and I can install all other Software from Aerosoft, Flight1 and so on without any issues?
Not possible to say, unless checking your system entirely for issue. But right now, the software that doesn't work is Ivap, not the Addon Manager so, the real question should be, why Ivap is not working now.
Maybe I will waiting for the time when I have to install my system and FSX comlitely new.
This will surely fix it so, once you'll do that, you'll realize both that the Addon Manager wasn't the problem, and that it usually works (AFTER you redefine the hotkey) together with the Ivap program.