erazor

Solved:

After upgrading I transfereed the blog and the database to a new server. After debugging the wordpress code down to the query:

$query = "SELECT $select_this FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id WHERE tt.taxonomy IN ($in_taxonomies) $where ORDER BY $orderby $order $number";

which is generated in wp-includes/taxonomy.php. Running the raw Query in mysql always resulted in a:

ERROR 1 (HY000): Can't create/write to file '/tmp/#sql_500e_0.MYI' (Errcode: 13)

According to my own research this is caused by the wrong write permissions on the /tmp directory. So the ultimate fix for my problem was this:

chown root:root /tmp
chmod 1777 /tmp
/etc/init.d/mysqld start

What a surprising solution and what a journey to figure it out.

Yes I intend to carry over the content.