meefa

Хоть и прошло более трех лет с момента вопроса, но решение оказалось очень простым и я не мог не поделиться.

Нужно указывать не абсолютный путь на 404 страницу, а относительный. В вашем случае должно быть
ErrorDocument 404 /error/404
вместо
ErrorDocument 404 http://dev.domain/error/404

httpd.apache.org/docs/current/mod/core.html#errord...

Note that when you specify an ErrorDocument that points to a remote URL (ie. anything with a method such as http in front of it), Apache HTTP Server will send a redirect to the client to tell it where to find the document, even if the document ends up being on the same server. This has several implications, the most important being that the client will not receive the original error status code, but instead will receive a redirect status code. This in turn can confuse web robots and other clients which try to determine if a URL is valid using the status code.