Make sure your copy of Internet Explorer, and any of your customers having a problem is configured to NOT "Show friendly error messages"
Issues
Internet Explorer by default is configured so that it doesn't display useful error messages from the Web server. This makes troubleshooting much more difficult than necessary.
Solution
Before installing Adobe Content Server and whenever a customer has a problem, make sure to disable this terrible feature.
- Internet Explorer > Tools menu > Internet Options > Advanced Tab > Browsing
- Uncheck the "Show Friendly Error Messages" option.
- (A better name for it would be: "Hide useful error messages".)