I have another problem that also yields '403 you are not allowed to access index.php' type error, when I try to get a preview on a post.
I tested around, and the malicious word seems to be "from". Just in the first sentence of the post it says: "[...] better accomplishments, aside from (quite literally) [...]".
If I cut the from out or make it italic, everything is ok. Otherwise, no preview, and no posting possible.
This is also true for this very post I am just posting, but strangely, just one week ago I could forge previews without issue, and I doubt I haven't used the word "from" somewhere in these huge text blocks. What a moody technology.