@vaclavelias
I have a new install of WP on IIS7 like you. I used the Web Platform Installer to deploy both the initial WP/PHP/MySQL install as well as the site.
The IUSR account was already set with Modify rights on the plugins dir for me. Si I would say it’s safe.
I also have this problem when trying to upgrade both plugins or WP (to 3.1). I’m using the latest WinCache version (Version 1.1.0630.0 Build Date Jun 29 2010 10:58:05). Disabling it do not correct the issue for me for updating WP or plugins.
This looks like an NTFS permission issue to me but PHP is all new to me too and I’m not sure what permissions are granted through the related services.
I’m going to use some monitoring tools to see what credentials are trying to write to the wp-content directories during the upgrade. I’ll keep you all posted.
