Thank you for your reply.
Unfortunately this is not a solution as I’m trying to understand how to change the Mautic’s own error pages.
When an error occurs, Mautic shows a custom error page with a generic message and with the mascotte.
I think this is something related to Symfony, but I’m afraid that acting on the Symfony configuration I will lose it on next update.
So, before doing anything, I’d like to know if the Mautic’s team has implemented any way to customize the pages directly from Mautic (via control panel r command line or custom script… I don’'t know)…