As the last resort maybe just refresh your pc (the windows 8 feature). The not so cool thing about it is that you will lose all your desktop or custom apps, metro apps will stay, and you will get a list of apps removed on the desktop. The refreshing will sure fix the problem. But I would try any other until doing that, or mby just do it right away if you don't have that many custom and desktop apps.
Still scanning... Will do that if this didn't work out: will I then lose all the "apps" on the start screen of MS Office 15?
During installation it didn't ask for a key: it was automatically done. So how to reinstall and adding your key?