There was an error during startup. Please see the Log for more details

@Moonized  Please try playing in a clean user folder: move the entire Sims 3 folder out of Documents\Electronic Arts and onto your desktop, and the game will create a new one the next time the launcher opens.  Don't add anything to it yet; just see whether you can get far enough to start a new save and play in live mode for a bit.

If that doesn't help, please let me know what kind of install this is: Origin, disc, Steam, or something else.

Base game and all packs etc are all on Steam, i deleted the whole sims 3 folder already a few hours ago frustrated hoping it would help but no. I just only downloaded the base game again on Steam to get a clean Sims 3 folder, i opened the launcer and the same issues came, ''There was an error during startup. Please see the Log for more details'', i also didn't get a new/clean Sims 3 folder. There is a Sims 3 folder but with nothing in it.

@Moonized  You'll need to delete any leftover registry entries associated with packs you've uninstalled; Steam doesn't do this on its own, or it didn't the last time I tested.  Hit Windows key-R, enter "regedit" without quotes, and back up your registry before doing anything else: the option is under File > Export.  Make sure to click "All" under Export range, in the lower-left corner, if it's not selected already.

Next, you can delete the relevant entries.  They're in two locations:

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Sims(Steam)

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Electronic Arts\Sims(Steam)

To remove an entry, right-click and select Delete.  To be clear, you need to remove both entries for any pack that isn't currently installed; removing entries for an installed pack will make that pack not load in-game but shouldn't prevent the game from loading at all.

If this doesn't work, please uninstall all of Sims 3, remove all the registry entries in the two locations listed above, restart your computer, and reinstall.  You can start with the base game only if you want to save time, then move on to the packs once you've confirmed that the base game works.

I'm at the step where i should delete the relevant entries but i can't find the two locations?

i've found the HKEY_LOCAL_MACHINE\SOFTWARE, but there is no WOW6432Node to be seen.. Am i doing something wrong?

I'm so sorry, I literally found it not even 10 seconds after i posted my last post. I'll keep on following your steps!