404 pages not working with yahoo hosting

I have my site installed in two places. Real one with yahoo and testing with siteground.

I realized that my 404 pages work with siteground but not with yahoo hosting. With yahoo hosting I get:

“Sorry, the page you requested was not found. Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.” (this is yahoo’s own 404 page, not mine).

My posts have the following link structure:
https://www.domain.com/index.php/y/m/d/post-title/

I’m not using .htaccess in either site (not allowed with yahoo). Any suggestions? (except moving to another host:)