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Error 403 when message includes HTML/CSS

I am a long-time user of phpList running on my own Fedora webservers. I have recently attempted to run it in on two separate hosted /cpanel services and had the same experience with both. After many frustrating days I have narrowed the problem down to this.
If I paste a plain text message into the ‘Compose message’ box attempt and to save and/or send a test message it works perfectly.
If I paste an HTML/CSS message into the ‘Compose message’ box and attempt to save and/or send a test message it fails with an error 403 and I can find no log file message to explain it. I don’t believe it is a path or permission issue. The version of php doesn’t seem to affect the outcome.
Help! Please!

@ajcocker It sounds as if there is some anti-malware software on your web server.

The usual causes of this include apache modsecurity (Modsec) and, if you are also running Wordpress, a Wordpress plugin called Wordfence.
Either of these might have a rule that is too strict. You will have to ask the server administrator to look at the web server logs for Modsec, but you can check Wordpress yourself if that is relevant.