[gderichti]

Ownership was the problem for me too. Running on Windows Server 2019 with IIS Version 10.0.17763.1

In my case, changing the owner of the upload folder to the useraccount of the AppPool did it. You can see which systemuser and how to apply this to your folder here:
iis – IIS_IUSRS and IUSR permissions in IIS8 – Stack Overflow