Hey there,

After a very long checking, we had found out that when there is this keyword 'data:' in the content, your server will restrict it.

Before we find out, we had go through the template, the plugins and the others one by one but it seems that it does not relate to the issue at all.

So we try to filter the content that you shared and finally we found out and confirm that as long as the content contains this keyword 'data:', your server will restrict the post data when you submit the reply.

Can you consult with your web hosting regarding this and see whether is there any content filtering for the post data on your server?

Thank you,
I really appreciate you taking the time to investigate the issue.
I have contacted namecheap then, will let you know.

No problem and please do keep us updated on this.

The guys at namecheap solved this for me.
Here is their reply

According to our check, there were a few triggered ModSecurity rules (server Web Application Firewall) which we have whitelisted for you.

We installed Mod_Security for increasing the security of our servers. It is an Apache module which works as a web application firewall. It blocks known exploits and provides protection from a range of attacks against web applications. Its activity may cause such issues from time to time, when requests sent to the server match with mod_sec rules. Sometimes such requests are false-positive.

Thanks again

Thanks for sharing this with us Niccolo