Hello,
My hosting provider is PlanetHoster.

I'm having a problem with my website, and to highlight the issue, I installed a "fresh" Joomla site.

I'm in the back office, logged in as superuser:

• I create/edit an article with underlined text: no error
• I create/edit a contact with underlined text in the additional information: no error

I'm on the front office, logged in as a "Manager":

• I create/edit an article with underlined text: no error
• I create/edit a contact with underlined text in the additional information: Error
  Clearly, the error is caused by a PlanetHoster firewall rule. If I disable it, I no longer get the error...
  Apparently, the HTML "style" attribute, introduced because I'm underlining text, is triggering the firewall rule, which sees it as a possible code injection.

We can all agree, the rule is flawed.

I tested it on OVH, no errors.

I did, of course, ask PlanetHoster... no response, other than "contact Joomla" :)

But my question is:
Why does mod_content (article) work in the front office but not in the back office, while mon_contact doesn't?
This suggests that the two components handle data entry differently. That mod_content doesn't trigger the rule, while mod_contact does.

Can anyone provide a reason for this asymmetrical behavior?

I can provide access to my test environment.