mudita08sri Please try playing in a clean boot:
https://support.microsoft.com/en-us/topic/how-to-perform-a-clean-boot-in-windows-da2f9573-6eec-00ad-2f8a-a97a1807f3dd
The one service to leave enabled is the EABackgroundService, which the EA App needs in order to run. Disable the rest as described.
When you reboot your computer, go through the Task Manager's background processes list shutting down any service that doesn't absolutely need to be running, for example anything from MSI Afterburner to RGB software might still be enabled. If you accidentally kill a critical process and it doesn't restart on its own, just reboot your computer again.
Don't open anything other than Sims 4 and the EA App while testing, not even a browser window.
The idea is to find out whether some other service on your computer is interfering with the EA App or the system's ability to connect with EA servers. If the clean boot helps, selectively reenable services until you get the error again, then narrow it down from there.
By the way, I would guess it's the case or you'd have said otherwise, but can your sister play on her account on her own computer? The idea being that this is definitely specific to your system.