This error occured to me, too. I fixed it by adding two lines:
putenv('TMPDIR='. ABSPATH .'tmp');
define('WP_TEMP_DIR', ABSPATH . 'tmp');
before
/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');
near the end of wp-config.php.
You have to make sure, that you have a tmp directory in your wordpress installation, that is writeable by the server.
Besides that WP changes HTML while displaying posts. I made a post (http://fthieme.net/blog/index.php/2007/01/25/emotional-apple-computers/) without the rich editor. Even in the database the post is correct as I wrote it, but when displaying it adds <p> and </p> tags. Suddenly on of the </p> tags is completely wrong. So the result is no valid (X)HTML anymore.
Same for me. http://blog.fthieme.net
Interestingly it looks good in konquerer but not at all in firefox. And yes I tried the default theme, too.
