Having issues setting custom 404 page...

Where do the web server logs indicate Apache is looking for the error pages? (Possibly very silly question but having been there myself, is the error page you're using what's actually triggering the 404 that you're looking at?)

FWIW and IMO, Console.app is far better at viewing the logs — short of using direct access to the files via less or zcat or grep — than is the Server.app log viewer.

When you've made the changes via Server.app, have you grep'd the configuration files to see where the settings were stored, or tried viewing the settings via the command line? Here's how this works, if you haven't sorted these details already...

At the lower level, the settings are stored in a plist and are accessed based on commands similar to this:

$ sudo serveradmin settings web

Using a "canary" string (xyzzy) as a target, here's where the error page is saved in the configuration data:

web:defaultSite:errorDocuments:500 = "/xyzzy.html"

I'd tend to expect this error page file would need to be relative to the root of the particular site.

The following is from some testing with the canary using the default site:

$ grep -R -i xyzzy /library/server/web

/library/server/web/Config/apache2/sites/0000_any_80_.conf: ErrorDocument 500 /xyzzy.html

The testing I performed before posting that was live, with no reboots involved, and with the web server running.

When looking for the "canary" amid the configuration plist data:

sudo serveradmin settings web | grep -i xyzzy

The recursive grep through the web server directories for the configuration file changes was shown.

I'd wonder if there's something else going on here.

There is my old favorite harmless diagnostic test for DNS services and local network weirdness, and then an Apache configuration file test for this case:

sudo changeip -checkhostname

apachectl configtest

Check your host name via the command, and test your Apache configuration, Launch Terminal.app and test your local DNS services and local network weirdness, and test your Apache configuration file syntax:

sudo changeip -checkhostname

apachectl configtest

That'll get a baseline for the common errors. Make sure you're not in .local for your server DNS domain.

Then go read your Apache logs. See if there are any errors being logged there.

If you're not seeing any the entries in the plist file that correspond to changes you have made, then I'd try rebooting and see if that unsnarls Server.app. Next up is probably resetting and reloading Server.app, on the off chance something has gotten corrupted.

I'll assume those two commands reported no changes required, and that the syntax was OK.

Just realized I might need to repoint the configtest tool, as the files moved and Server.app doesn't use the copy that's in the default location:

$ apachectl -f /library/server/web/config/apache2/httpd_server_app.conf -t

Above is the same configtest, with the path specified.

Details:

cat /library/server/web/config/apache2/readme.txt

The command-line command used to change the settings is the same as used to list it: serveradmin. That's the path into the various plist files. For details on that command, use:

man serveradmin

The Apache configuration stuff is all underneath the directory:

/library/server/web/Config/apache2/

but that shouldn't need to be edited directly, based on what I've seen with Server.app. When it's working, of course.