XF 1.5 Cant post with formatting

[Travis]

My site suddenly has an error with posting. If I write a post with no formatting, it posts fine. The moment I introduce formatting, even just changing the text colour, i get this:



I am a few versions behind the latest release (running XenForo 1.5.10), would that be causing it? Cant quite afford to renew my license at the moment

 

[Paul B]

That's a web server error, typically caused by mod security.

Contact your host and they should be able to resolve it by whitelisting the rule or disabling it.