The Sims 3 opens and closes without something happening and no error warning

@t0nai  That does sound strange.  For now, leave Origin out of the picture.  (You don't have to uninstall it, just don't open it.)  Under System Preferences, check your settings in Security & Privacy to make sure that your firewall is off (third tab) and you're allowing apps from identified developers as well as the App Store (first tab).  Mojave's security features are more strict than those of its predecessors, but TS3 is certainly still playable.

If this doesn't help, try launching the game while your computer is completely offline.  Since you haven't been able to open it since your reinstall, you won't get very far.  I'm just wondering whether this will change anything.  If you get an error message, please post a screenshot.  You can grab one by pressing cmd-shift-4 and dragging your cursor over the section you want to capture.

Oh, also disable iCloud temporarily.  I have no idea if this has anything to do with it, but the game closes exactly when it would be generating a user folder, the one that lands in Documents.  Maybe iCloud changed its settings when you updated to Mojave; it's been known to do so.  If it's trying and failing to sync or upload Documents to cloud storage, that might explain the problem.

Let me know, and I'll try to figure out what's going on.  And I'll definitely ask Bluebellflora for her thoughts too, when she has time.

@puzzlezaddict Unfortunately, none of the above helped. I changed my security settings, logged off my iCloud, deactivated my Wifi connection and left origin unopened. Still, the Sims icon appears in my dock for a moment, then the application closes again. 

@puzzlezaddict I tried running the game on a different user on my Mac. The „unknown error“ message popped up. Then I tried bypassing the launcher by changing the info.plist file. A new error showed.

„The Sims 3 - Unable to start game. Service initialization failed (0x0175dcbb)“

@t0nai  That's actually good news; the unknown error we understand and can usually fix (not always, but usually).  Is there anything different about that new user account, like is one of them admin and the other not, or are the settings different?  This is for my curiosity and future troubleshooting; feel free to leave the question alone.

In your case, with only the base game installed, you'll need to edit two .sgr files in TS3's program files to force the game to recognize your graphics card.  Some iMacs come with dedicated graphics cards, and others don't.  The specs you listed refer to an integrated card, but if you have both, the dedicated card might not have been listed in the initial specs.  So please go to About This Mac again, then click System Report, then Graphics/Displays, and let me know which card(s) you see listed.  Once I know which card you have, I can send you instructions for the edits, or a link to instructions if Bluebellflora has already prepared the text for your card.

Also, undo the edit you made to bypass the launcher.  That can come later, if you want.

I only have the integrated Intel Iris pro 1536 MB card.

This is what my report says (unfortunately in German, but I guess the most important stuff is understandable):

  Chipsatz-Modell: Intel Iris Pro

  Typ: GPU

  Bus: Integriert

  VRAM (dynamisch, maximal): 1536 MB

  Hersteller: Intel

  Geräte-ID: 0x0d22

  Versions-ID: 0x0008

  Metal: Unterstützt, Funktionsset macOS GPUFamily1 v4

  Displays:

iMac:

  Displaytyp: LCD

  Auflösung: 1920 x 1080 (1080p FHD - Full High Definition)

  UI sieht aus wie: 1920 x 1080

  Framepuffertiefe: 24-Bit Farbe (ARGB8888)

  Hauptdisplay: Ja

  Synchronisierung: Aus

  Eingeschaltet: Ja

  Rotation: Unterstützt

  Helligkeit automatisch anpassen: Nein

  Verbindungstyp: DisplayPort

Last time I checked Bluebellflora only had instructions for the late 2013 21“ iMac (iMac 14,1) with the Intel Iris Pro 5200

@t0nai  You're right, the German isn't an issue; all the important information is easy to find regardless.

You do in fact have an Intel 5200, even if it's not listed that way.  I checked Bluebellflora's files, just to be sure, and the device ID she lists matches yours, so it should work fine.  You can download the text to add to your files here:

https://bluebellflora.com/2018/07/24/sims-3-mac-players-i-need-your-help/#Filesconfirmed

Be sure to test in the alternate user account, since that's the one that's being more cooperative, relatively speaking.  An unknown error is better than no response at all.

I changed my .sgr files, but I still don't get the game to work. On the alternate user I get thte "Unknown error" message, and on my main user nothing happens.

@t0nai  Can you post screenshots of the edits you did?  Sometimes TextEdit messes with the formatting.

And I'd also like to see part of your deviceconfig, if you have one—just the entry between Graphics Device Info and Options, so from about 25 to 40 lines down.

This is what my edits look like. I'm not quite sure where to find the deviceconfig.  (I still don't have the The Sims 3 game file, because I could never launch the game after reinstalling)