Those errors result from a false positive detection, when the content being posted happens to match one of the unknown but probably arcane content filtering rules that our hosting service has recently put in place.
We presume that the rules are intended as a defense against certain types of attacks against websites, for example "cross-site scripting" malware.
However, sometimes the rules match things that are in fact completely innocuous. As an example, it turns out that at this moment the sequence
<newline>Move<space>
results in a "403 Forbidden". Apparently it's forbidden to move! Or something like that... (I have no idea what the rule might be intending to defend against.)
Anyway, if you get one of those "403 Forbidden" errors, the only known solution is to use Preview, in combination with something like binary searching your post content, to find the snippet that's causing the problem and change it to something that does not cause the problem.
In the case of <newline>Move<space>, changing the text to
avoids the 403 Forbidden, while producing exactly the same output.<newline><space>Move<space>
When you get caught by one of these, please let Admin know what you ran into. Sending email to administrators@photomacrography.net is a good method of sending stuff that you just can't make work. Don't try using the Contact form. That will have the same underlying problem, but with an even more confusing message: "Form PHP script is missing from web server, or PHP is not configured correctly." (Sigh...)
--Rik