This is what I had to do on my Ubuntu 9.10 server to get the 'own' error documents to show.
Edit the file
Scroll down to the section
# The internationalized error documents require mod_alias, mod_include
# and mod_negotiation. To activate them, uncomment the following 30 lines.
Alias /error/ "/usr/share/apache2/error/"
Then change the Alias line to something like...
Alias /errors/ "/usr/share/apache2/error/"
So that the alias is something other than /error/
Save the edit then reload Apache
Then if you have selected 'own' documents in the create site section of the ISPConfig you should get the error documents read in from /web/error/
If you untick this 'own' document box and save (then wait a minute or so) you will get the normal apache errors.
Maybe someone will come up with a way to retain the localized language pages because my kludge will probably break that bit. But it works for me. I make everybody read English.