I tried contacting EA support but (as expected) they couldn't help me either. After googling for HOURS I found a fix for this, but it's pretty complicated. I followed the instructions in this link https://answers.ea.com/t5/Bug-Reports/DipStaged-game-files/m-p/12310842 (downloading python, running the script etc.). I had to run this fix a few times until I noticed the launcher still tried to download one specific file in an endless loop. The file is called "SASFullBuild2.package" and is located at The Sims Medieval\GameData\Shared\Packages. So I guess the download for this one file is broken. I asked a friend of mine, that has The Sims Medieval installed, to send me this one file and voilà, it worked, I can play the game again.
Your method seemed to work, but as you said the SASFullBuild2 is insistent on being downloaded. Would you be somehow able to provide working file?
