Ciao Fabio,
This is clearly related to VirtueMart, not the Stripe plugin. It may depend on which combination of versions you are using (VirtueMart, Joomla, PHP, ...) and maybe your host set up.
First, please check and fix the error in the debug message:
Key folder in safepath unaccessible
From the
VM panel >
Configuration >
Templates, check the
Safe Path field, and the message and suggestion at the top of the page.
Check if the Safe Path error and PHP warning (
mcrypt_encrypt() key of size 0...) are gone. If not, then step two, check the server logs about the mcrypt() error, and maybe other errors that could be related, and contact your hosting provider for details. Some of them disable or tweak the PHP libraries for their own needs.
Also, you can google and look at the VM forum. There is clearly a bug that was fixed between PHP 5.4 and 5.6, and the mcrypt() behaviour is different from a VirtueMart version to another, see
https://forum.virtuemart.net/index.php?topic=133436.0.
If there is no reason to keep an out of date PHP 5.4, I strongly suggest to update to at least the latest PHP 5.6. And same with every extension and components used for the website.
Please let me know how it goes.
Kind regards,
Hervé