Unable to export - browser downloads export.php with error

[ggr88]

I'm running 5.0.0c and on clicking export for any records the browser tries to download export.php and inside is error message:
<b>Fatal error</b>: require_once() [<a href='function.require'>function.require</a>]: Failed opening required '' (include_path='/.../sugar/include/..:.:/usr/lib/php:/usr/local/lib/php') in <b>/...../sugar/include/export_utils.php</b> on line <b>144</b><br />

Anyone know what the cause of this is?

Update: Just found export was working fine in IE7, but in Firefox with Free Download Manager was kicking and and causing this. Cancelling on the first FDM popup window then gave another window with the .csv file.

Thanks,
Gareth