Thanks for the response.
Actually, the BEX01 does not appear to have a " preview_iframe.tpl " file. The temporary workaround I ended up using was to edit the $SERENDIPITY_HOME/serendipity_config.inc.php file and set the value: $serendipity['use_iframe'] = false;
I don't get the full preview with ...

I have recently upgraded from 1.4.1 to 1.5.2 and am using the BEX01 theme and old and new blog entry previews have also stopped working for me. I haven't done any customizations of the BEX01 theme or the Serendipity installation.
I am not seeing any errors when I click on preview - just a thin green ...