I have been getting the same error trying to edit a CC forum message.
We have a thread with a single message that I created in May, 2014. The message is nothing more than a bunch of useful links within tables (to make it look nice ... they don't allow <STYLE> tags for CSS).
I update this message several times per week. It worked as recently as Thursday. It did not work on Friday. I figured that it would be a temporary thing but when it occurred again on Saturday, I contacted CS.
So far, they have not been able to help me.
There is an incident ID on the error page which would be helpful to WGT if somebody there would log in to their Incapsula account to see what's causing the problem.
After some experimentation, it seems that the problem is a result of a a new size limit for HTML code. I can load up so much and edit the message without any trouble but if I add another bit of innocuous text, I get the error.
Scott, you might want to do the same thing (i.e. add a bit of nothing) to your HTML and see if you get the error again.
If I learn anything from CS, I will relay it here.