There's often a simple explanation for the failure to display a requested webpage. It may be as straightforward as the file or files don't exist, or access to them is denied because of insufficient permissions. In this latter case, we'd usually recommend file permissions are checked, assuming that page is meant to be accessible, of course.
'Triaster Server 2011 - Folder and File Permissions'
Quite recently, a customer reported that users were being prompted to log in when trying to open webpages. We knew that the file permissions had been revised on that system, so as a first step, we recommended that those permissions were checked. Nothing amiss was found. The cause was identified with the help of Fiddler, a free web debugging proxy.
This is a powerful utility, but you don't have to be an expert to find it useful. It's installed on any PC from which web requests are to be monitored. If investigating issues with a Process Library website, this could be run on any PC used to view the Library.
In this case, attempting to open a map's webpage revealed requests to files that didn't exist, something like this.
For one of the missing files, the remedy was simply to find an instance of it elsewhere in the installation, and copy it into place. For the file depicted above, there was an additional factor, which is described in this article.
'Unexpected Authentication Prompts'
It's still not properly understood why a missing file led to a prompt for authentication. It was thought that a redirect to an error page for which there were insufficient permissions might have been the cause, but this wasn't proven.
That's quite a simple example of how Fiddler can identify an issue, even to those with little technical knowledge of web requests. We've also used it to verify authentication methods when securing a Process Library website, but that's a little more complex, and beyond the scope of this article.