I use Mailenable Pro version 2.091 for mail. I just recently started getting a strange error when I try to access its webmail applications:

HTTP Error 401.1 - Unauthorized: Access is denied due to invalid credentials.
Internet Information Services (IIS)

This is very strange because up until a couple of days ago it worked just fine. No complaints ever.

I used Explorer to check all my permissions. I found a list of required permissions for IIS 6.

The first thing I noticed that was out of the ordinary was in the folder <windir>IIS Temporary Compressed Files . The page I found above said that Creator Owner needs Full Control over this folder. However, when I granted that permission, and pressed Apply, all the checkboxes that I placed in the Allow column just disappeared! Anyway... Moving on.

In the folder <windir>\system32\inetsrv\*.vbs Administrators had to be granted Full Control. I had 3 vbs files in there, clusftp.vbs, clusweb.vbs, and iis_switch.vbs. You see the permissions for all of them at once if you highlight all three, right click, click properties, click security tab.

When I got down to <windir>\system32\inetsrv\History, I found that neither Administrator nor System was granted any permissions to this folder when they should both have had Full Control.

I decided I was on the wrong track and switched gears for a little bit. I turned to the system logs and found that my AVG Antivirus was throwing a application log error every few seconds when it was running. This is getting stranger by the minute and every minute is getting later at night. I sent an email to AVG's support and called it a day.