If keyword lookups can not be found, pages can not be displayed and the error messages are not the same as if the Notes client was returning the error. Keyword lookup failures can stem from hardcoded server references or database references which can not be access under the URL provided. There are two things you can do to help make troubleshooting easier.
1. If you are using IE, there is an option under the Internet Option, Advance tab, for “Show friendly HTTP error messages”. Turn it off and restart the browser.
2. Make use of the $$Return… error messaging; there are four.. Do a search in designer help for “$$Return*”, you should find the “Displaying a customized error message on the Web”; it will list them all.
Hope this helps.