trouble after upgrade to 2.3

Oh my god, i have a major problem i cannot fix. Hope you guys can help me.

I upgraded to 2.3 by the book, backup, disable plugins, all the things. After installing the new files, i ran the upgrade script. Which work fin, it says the BD is fully updated, so all fine, i thought.

But now, my whole blog is empty on the frontend. Although i can edit all my posts and see all the comment on the wp-admin side, the blog self dis broke. It shows the standard 404 page from WP.

When trying to view an post of page of archive i get the following message:

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

The WordPress install stand on https://www.buxx.nl
For example of error: https://www.buxx.nl/blog/contact/