Due to the server change of the IPS platforms I manage, it is no longer possible for me to embed content (Youtube but also internal to the forum) in posts, pages and anywhere on the Forum.

I was told that it could be a problem related to security and in particular to a rule in modsec (mod security).

Is there anyone who could give me clues about this so that I can contact the hoster with accurate information?

To add some more details to this: the problem are POST requests that contain <iframe> elements being blocked by the server. Like YouTube embeds as mentioned above.

I would generally recommend to ask your host to disable mod_security for your website.  They can turn it off for your site without effecting other sites on the same server if you're on a shared host.  If you're on a VPS, it would be even safer as there is no possibility of other impact.

Generally mod_security causes more problems than it fixes in my opinion.