Hello and thank you for replying!
I have tried these steps as well but unfortunately the same result occurred.
I have attached some screenshots too - as you can see, I have created a new world state where I included the hero I want to delete (one of the 4 that are "stuck" and can't be deleted); edited all the choices up until Inquisition, saved, logged out, cleared browsing data, logged back in, tired to delete the hero, got error.
I only tried this in Edge this time, but I seriously doubt that there will be any luck in another browser as I tried these same exact steps prior to raising this topic, on a different hero from one of these 4 and I got the same result (when I tried that, I did it in Chrome)
So basically the only thing left now if for the team to delete those stuck heroes? (it seems that the issue is with DAO, I can delete heroes in DA2 and DAI without any problems). Also, those heroes were created some years ago, when I was able to actually log into the EA Servers, whereas now I'm no longer able to do so (probably the reason why the character that I have created for my most recent playthrough from 1 month ago did not sync in Keep at all, along with the achievements, and I had to manually modify my active world state to reflect the choices I did, so that I can import them into DAI).
I purchased Dragon Age Origins Ultimate Edition through Steam, along with the other titles, and played the games so far through Steam (Steam account is successfully linked with EA App as well, as I have checked this when trying to troubleshoot this problem).
But these 4 particular heroes that I can't seem to be able to delete, were created when I was running an instance of the game that was acquired for free through Origins (the old EA App) - free week or something of sorts - back in 2015-2017.