File upload error. Could not I have Drupal 8.4 running on Windows Server 2016, and when trying to use an upload field on a content type I get this error

The file phpC128.tmp could not be saved. An unknown error has occurred.
File upload error. Could not move uploaded file.

I deleted the files folder and IUSR, account PHP is running under, recreated it for this month as expected, but this still errors out. I see the file in c:\windows\temp but it doesn't make it to my site folder. IUSR has full control to the files folder and the 2018-06 folder.

In IIS, the AppPool is running under ApplicationPoolIdentity as well.
Moving the file path from `c:\windows\temp' to another location, and after setting the IUSR permissions on that new folder, it worked as expected!