404 errors with checkout and user login

[David B.]

The whole prestashop seems to be working fine, but suddenly 404 errors occur if you click "checkout", "Log in", "Your account", or even just "cart". The message displayed looks like this:

The requested URL /webshop/order.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

The file permissions for order.php are 644 and I installed everything in a subdomain called "webshop".

It also seems to instantly forget that there is an item in the cart (which the user just added to it before checking out).

I did make a front end user for myself to experiment with.

Also, I don't see a "register here" button anywhere on my webshop. Is it in a separate module (that I might have accidentally disabled), or does it just ask to register after you click "Log in"?

Thanks in advance,

David