It is not possible to get Ae to work. Uninstalled, installed, same problem.
Without any actual system info we can't really tell you much. From permissions issues on the folders to technical problems with the font rendering this could be anything. The first thing to do would be to delete the Adobe and Windows font caches in safe mode and then also check the graphics driver, system DPI setting and so on. also of course weed out any unnecessary fonts you may have added. A single "bad" font can totally mess up things.
Thank you, Mylenium.
After trying various things, I had the idea this morning to start Ae with administrator rights. And see, it starts up. And then it also starts without administrator rights. But that didn't solve my real problem: Premiere needs Ae in the background when using Motion Graphics and still claims that Ae is not installed (it's running right now, as you can see).
Can you try changing the permissions as instructed in the following article: https://helpx.adobe.com/ie/x-productkb/multi/preferences-folder-read-only-error.html
If it doesn't help, create a new user account (for testing) and see if After Effects is working in it.
Let us know how it goes.
The problem is that Ae itself is running. However, I have checked/set the permission in all the folders, but Premiere still reports the following:
1. Shows progress bar at 15% for ~1 minute
2. then this: