Upgrading to php 5.2.3 and moodle 1.6.5 works too. I had this problem with one of my test servers running moodle 1.6.1, upgrading php 5.2.2 to 5.2.3 did not solve it but when I upgraded also moodle 1.6.1 to 1.6.5 forums started working.
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Forum problem after PHP 5.2.2 upgrade
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