Error 0x803f81f5 is solved by downloading DLC from an additional Microsoft account.
Detailed guide:
- Sign out of your current account in the Microsoft Store app
- Enter your activation key in the box below to get your account details to download the DLC.
- Log in to the "DLC download account" in the Microsoft Store app on your PC using the login and password obtained above.
- Go to the browser link https://account.microsoft.com/devices/content and log in to the same account.
- Before proceeding to the next step, make sure that Microsoft Store account is the same as your account on the site!
- Click to remove one of the devices as shown in the screenshots below.
- Do not delete devices unless you are signed in to forzahorizondlc_**@denuvogame.com in the Microsoft Store app (take login and password from point 2)
- No need to delete all devices. It is enough to delete just one of 10.
- Try to download DLC again
- After all the DLCs are downloaded, you can go to the main account with the game.
If the error persists, then you did not complete step 3.