Write Permissions Problem sitemap generation with Google Sitemap

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Hi everyone, I've a problem with the Google Sitemap 4.2.0 module.

Despite trying to uninstall and reinstall the module and also trying the previous version, I still get this error when creating the sitemap:

An error occured while trying to check your file permissions. Please adjust your permissions to allow PrestaShop to write a file in your root directory.

I'm using Prestashop 1.7.6.8 and Google Sitemap module 4.2.0

Prestashop is installed on a VPS with Apache and Mysql.
I've already tried to change the root permissions to both 775/644 and 777/777. Also I've tried to change the owner from www-data/www-data to root/root, but I keep getting the same error message.

I also tried manually creating a sitemap.xml file under the main root and giving the permissions 755 or 777, but the file is not rewritten.

Do you have any advice for me? I don't really know where to turn anymore.

Thanks a lot right away, I hope you will be able to help me.