Hi,
The file paybox_8.php is created at the root of your website when the settings of the payment method are saved in the backend.
If you have a 403 error on that file, it means that either that file couldn't be created because of access rights, or that it got created with invalid access rights.
So please check whether you have that file. If you do, fix its rights, if you don't, fix the rights of your root folder and save again the settings of your payment method so that this file can be created.
It should then work fine.
We have to do create that file as paybox limits the number of characters of the return URL. Without that file, the URL, which would include the different parameters necessary for the SEF system of Joomla to direct to the payment plugin, would be too long and refused by Paybox.