by Daniele Cordella.
I found a solution.
I annotate it here in the hope it may help people facing the same issue in the future.
Following the suggestion from: https://www.cyberciti.biz/faq/apache-limiting-upload-size/
I added to my apache vHost:
<Directory "/path/to/my/moodle/instance">
LimitRequestBody 0
</Directory>