User unable to open archived item in Outlook ?

On the Enterprise Vaul Server, it looks like the Default Website has a domain and ip restriction set
Check it to see if its set to deny people access who aren't on that list

It also could be that something like ASP has been set to not run in IIS

can you open up an IIS log on your EV server and do a search for 403 and paste on of the lines here? along with the very very top line of the IIS log that has the headers?

Once you have that line, you will have a substatus code for the 403, you can find the definitions here

http://technet.microsoft.com/en-us/library/cc737788(v=ws.10).aspx

"Typically, 403 errors occur when an operation or request is disallowed because a requirement other than proper authentication credentials is not met. Commonly, this error occurs when a script request is made to a Web directory for which script access is not enabled. In such a case, first verify for which resource the request is being made. If the request is, indeed, for script code, such as ASP or ASP.NET, check the execute permissions for that directory in IIS Manager and ensure that at least Script is selected."

If its 403.6 or 403.8 then its because the IP address or domain name has been blacklisted OR isn't white listed

If its 401.1, 401.2 or 401.3 then its NTFS permissions on the EV install directory

 

Well, I'm the domain administrator, but how come this affecting consistently across the whole company ?

Jesus, as can be seen from the following screenshot, it is not applicable:

it doesn't matter if you're the domain admin or not
if you have a white list saying only accept requests from 192.168.100.10 and you're accessing from 192.168.100.11, it doesnt matter whether you are domain admin, backup admin etc your IP addresses don't match and therefor you won't be able to access

If something like the ASP ISAPI filter has become Disabled or not allowed to execute then IIS simply won't run any ASP pages at all, and if NET gets disabled then same for .aspx files

expand out Web Service Extensions, make sure Active Server Pages is set to enabled

Yes they are all enabled already.. ?

Yes, I have changed that into Granted Access, but so far it doesn't work ?

find the 403 sub status from the IIS logs
also make sure that you put Granted access on \EnterpriseVault and not juse Default Web Site etc