The Apache Modul Atomicorp (asl) seems to block the access to the binary. Maybe it helps to copy the binary (I think you meen the imagemagick convert?) to a other location and set the path in config.

I run into the same issues with an installation of serendipity on nginx. I got some 404 Errors.

The Relative HTTP Path was not correctly set while installation (conf was set to /serendipity_admin.php/).

I have used the following nginx rewrite rule, for a workaround

rewrite ^/serendipity_admin ...