While installing Agents4 for IIS on a Windows 2012 Server R2, I experienced an error which has held me up for a couple of days. It may be worth adding my experience to the documentation we give to customers.
Part of the couple of days was given over to getting the installer to actually tell me why it wasn't installing (previously it didn't even report a problem and seemed to have succeeded). This particular problem now manifests itself (with the revised installer) as:
This is fixed by starting up IIS Manager and selecting the Configuration Editor under the Management section:
When the Configuration Editor is open, there is a drop down towards the top of the screen:
Every single one of the items in the drop down list must be visited and and "Unlock Section" clicked:
Fortunately this isn't quite as tedious as it sounds. When the drop down box is selected, pressing the down arrow key is sufficient to select the next item in the list. When all elements have been visited and all sections of the file are unlocked, the installation can be attempted again.