Fatal error in newthread.php after 3.6.6 upgrade

Initial report made here.

I have now copied my live system sideways to make a test system. I have uploaded the 3.6.6 files over the top of the test system and performed the upgrade. No errors occurred during the upgrade, but I am unable to create new threads or use the quick reply - although the normal reply option works.

If I try to create a new thread I get "Fatal error: Only variables can be passed by reference in /home/mkivsupra/htdocs/test/newthread.php on line 183"

If I try to use the quick reply I get "The following errors occurred when this message was submitted" but it does not show any errors.

I have disabled all plugins using both the option in the admincp and by adding define('DISABLE_HOOKS', true); to the config.php

I have created a new template set named "Clean 3.6.6 Template Set" which has no parent template.

I have also replaced my upgraded forum folder with a completely clean version so that the "Suspect File Versions" option now reports no unknown, changed, or incorrect version of files.

Depsite this, the problem still persists.

EDIT - I am running PHP 5.0.5 and MySQL 4.1.20