If you click on the 'Forum Posts' tab for a user you get an error - 'Course id is incorrect'.

I found the problem to be the incorrect usage of optional_param() in mod/forum/user.php

namely, starting at line 7:
$course = optional_param('course', PARAM_INT);
$id = optional_param('id', PARAM_INT);
$user = optional_param('user', PARAM_INT);

This is passing PARAM_INT as the default return, instead of as the type.
Should be:
$course = optional_param('course', NULL, PARAM_INT);
$id = optional_param('id', NULL, PARAM_INT);
$user = optional_param('user', NULL, PARAM_INT);