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 the 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. The account has full control to the files and 2018-06 folders.
In IIS, the AppPool is running under ApplicationPoolIdentity as well.