My theme vendor has his own custom error pages. I need to disable them in order to use JoomSEF's custom error pages.

The theme vendor suggested asking JoomSEF support first. Could you please help ?

Hi,

There's no need to disable any custom theme error pages. JoomSEF displays its Custom 404 Page like the standard Joomla article.

Unfortunately at new.sailingreekwaters.gr it loads the theme 404 page. Any idea?

Hi,

This can happen when you have "Parse Joomla SEO links" option enabled in JoomSEF's Advanced Configuration and you access some non-existent URL which JoomSEF sends to Joomla for parsing and Joomla parses it only partially. In such case JoomSEF doesn't know that the URL doesn't exist and Joomla itself displays the standard error page.
Try disabling the "Parse Joomla SEO links" option in JoomSEF's Advanced Configuration. Please note that it will cause old style Joomla SEF links stop working, so if you use Default Joomla router or a Component's Router as active handler for some of your components, their URLs won't work properly.

If disabling the option doesn't help, it's probably caused by some configuration of your web server or some 3rd party component or plugin installed in your Joomla.

i tried disabling the "Parse Joomla SEO links" but no luck. Any other suggestion ?

In that case I don't think this is related to JoomSEF. It's either caused by some of your server's configuration or the template you use in Joomla.
We can of course check this for you as a paid support service if you're interested. For pricing please see: www.artio.net/e-shop/support-services/joomla-support-1h