Symptom: ErrorDocument directives in .htaccess files under addon domains do not function or generate internal server errors when there are one or more rewrite directives in the .htaccess of the parent domain and no overriding rewrite directives in the addon domain .htaccess.
Cause: The rewrite directive at the top level is attempting to rewrite the requests to the addon domain, as .htaccess files are recursive to subdirectories.
Solution: Place a directive at the top of the .htaccess in the addon domain to disable the rewrite function:
RewriteEngine Off