WordPress unable to create folders even though correct NTFS-rights are set

We are trying different NTFS-rights settings on a IIS-webserver, and we removed the Users-group from the folder containing all the sites.

We then ran into the problem that a WordPress installation could no longer create folders for uploaded files. (The setting to organize uploaded files in folders according to year/month is in use). The files are stored on a different site, a mediasite. (Within the same IIS-server)

If we manually create the folder it works just fine to upload files again.

Naturally, we reversed the rights so that the Users-group once again had read-access on the hosting-folder, but the WordPress-installation can still not create folders.

We even tried giving Full Control to both a few different groups, as well as explicitly to the users that run the sites.

I'm at a loss, and no longer sure this is a NTFS-rights-problem, since running Effective Access clearly shows that the users should be able to create folders, as well as traverse out of it's root, and into the other site (mediasite).

After every change, the sites (WP and mediaserver) have been restarted in IIS.

So my question is, what else could have gone wrong here? What have we overlooked?

If anything seems unclear, feel free to ask and I will clarify.

Thank you!