Problem with installing Into the Future ("Showtime is already installed")

Yes, the title is correct.

I had all expansion packs except Showtime and Into the Future installed and I could play them without any problems.
Today I uninstalled Island Paradise and installed Showtime. So far things were okay.
Then I wanted to install Into the Future, but I get this pop up every time when I try to install it in my usual folder:



Into the Future isn't installed and I don't even know why it talks about Showtime? I uninstalled Showtime multiple times now (including via regedit) and reinstalled, but I always have the same problem.
I made sure Showtime is installed properly and it is - The game works perfectly fine.

Can I just install Into the Future in a different folder and move it to my usual folder? Or any other tips and tricks?

Switch over to the Sims 3 via ZLOrigin

But I just see that Showtime is installed in Eigene Dokumente\Electronic Arts\sims 3 which is of course utterly wrong.
I guess you now have in Eigene Dokumente\Electronic Arts a folder sims 3 and Die Sims 3. Delete sims 3

I would have preferred not to use Zlorigin because when I tried it a while ago for TS4 it was way too unreliable and not as easy to install as the "normal" way.

But I'll give it a try.
Unfortunately I'm already stuck. I follow this guide https://games4theworld.forumotion.com/t30092-zlorigin-made-simple#175577 but when I want to click on Zlorigin icon and want to log in it doesn't let me log in (tried as admin as well). It looks like the normal Origin log in page btw (I don't know if that's even supposed to be like this, but I don't have the normal Origin installed), and my password and mail from ZClient don't get accepted.
I don't know what's going on and can't find a solution anywhere.

check yhe taskbaar to see if you are already logged in. You can run into problems when you try to open it twice.