Why does my WIN+X menu not open anything?

Luke, use the Language Packs!

Ok, this is a solution which I came on testing something completely different. I just wanted to download the English language pack for Windows 10 (I'm a German user), just to see what exactly changes and how good Windows 10 can switch.

After I switches my language pack, I've got another language on my UI (wow!) and also my WIN+X menu worked again!

So after that I switched back to German, which made the UI and the menu German again - but not broken. It works now.

TL;DR Switch to another language pack, reboot, switch back again, reboot. Done.

Try the Win+X Menu Editor and see what it says about the existing entries.

Also boot in Safe mode to see if an existing application blocks the menu (for example the airdroid windows client is reported to do just that). If that's the case, you would need to identify that application.