mod_rewrite is not activated.
One of the modules, which our shop uses, is running Cron jobs to sync inventory - that is why it is backend related - I can see the that link in the logs of the Cron job. Unfortunately I am not able to change it in the Cron job settings, so I wonder if it is possible to enable those backend related link to work with language prefixes like /es/, /en/.
To give you a better view:
https://theshopname.com/index.php => redirects me to the homepage (301 and redirect to the /en)
https://theshopname.com/es/index.php => redirects me to the 404 page