Can't create or load saves, "Serious error"

@hugsohugs  The fact that the game works fine in the new Windows account means that the problem lies in your main account, and that it's likely borked permissions or some user setting.  For the permissions, that's why I asked if you were using an antivirus: these programs can block another app's access to Documents or a folder inside.

From here, one thing you could try is removing the Electronic Arts folder, in case the issue is that folder and not all of Documents.  If that doesn't help, the takeown command might.  (You do need to run it from your main account, not the new one.)  This is the command:

takeown /F "C:\Users\username\Documents"

Write your actual username for this Windows account (your main one) instead of username.  Since the syntax needs to be exactly right, please copy this command to a Notepad file, substitute in your username, and then copy that string when you're ready to use it.  Hit Windows key-X and choose either "PowerShell (Administrator)" or "Windows Terminal (Administrator)," whichever option is offered, and paste the command into the window.

Just to be thorough, please move the Electronic Arts folder out of Documents, if you haven't already, before trying to play.