I've had a look into my Vhosts_ispconfig.conf - and saw that the individual error pages were activated, although I didn't enable it in ISPConfig. Playing with the option didn't change anything (but, yes, the file was rewritten). Installed release is 2.2.0.
Alias /error/ "/var/www/web1/web/error/"
ErrorDocument 400 /error/invalidSyntax.html
ErrorDocument 401 /error/authorizationRequired.html
ErrorDocument 403 /error/forbidden.html
ErrorDocument 404 /error/fileNotFound.html
ErrorDocument 405 /error/methodNotAllowed.html
ErrorDocument 500 /error/internalServerError.html
ErrorDocument 503 /error/overloaded.html