Forum OpenACS Q&A: Response to How do you customize the error pages from AOLserver/ACS?

You need to set the following the config file:

#
# Internal redirects
#
ns_section "ns/server/${server}/redirects"
ns_param 404 "/global/notfound.html"      ;# Not Found error page
ns_param 403 "/global/forbidden.html"     ;# Forbidden error page
ns_param 500 "/global/servererror.html"   ;# Server error page

If you want to use templating then you will need to modify the request processor.