I have seen overly restrictive mod_security rules that flag the content of some posts. If that is the problem then you need to ask your host to relax the rules. If you can narrow it down to a specific word or phrase that is triggering the error then that will help your host.
I've been in touch with our host and they've disabled the above "hidden text" mod_security rule and the problem goes away. They turn it back on and it comes back so it's definitely just the above rule.
Our hosts are reluctant to permanently disable the rule (but will if we require it). I'd rather try to find a solution and work out which bit of our code is causing it. It looks like it's the "width:0px" bit of code which we have in a few places but the fact it doesn't happen for all users makes this much more difficult to track down.
With some host you cannot disable mod security via the htaccess file. Some require it to be done at server level (their end). So you might have no option but for them to do it for you, as doing it via XenForo htaccess may make no difference at all, with it still being active.