Upgrade to debian testing breaks smarty sites

I've had a rather catastrophic failure upgrading from Debian stable to
testing (ie to PHP 4.3.3 and apache 1.3).

I now receive a "Warning: Smarty error: unable to read resource:
"index.tpl" in /usr/share/php/smarty/libs/Smarty.class.php on line 1088"
when accessing the home page.

The templates are in the same place as before, but it seems they can no
longer be read.

Any ideas much appreciated!

Benjamin