Getting the following suddenly on Forum List requests.

We encountered a problem. The reason reported was

Script: /home/virtual/site12/fst/var/www/html/ubbthreads/scripts/cfrm.inc.php
Line#: 163
SQL Error: Can't create/write to file '/var/lib/mysql/#sql_421d_0.MYI' (Errcode: 13)
SQL Error #: 1
Query: SELECT c.CATEGORY_TITLE,c.CATEGORY_ID,c.CATEGORY_DESCRIPTION FROM ubbt_CATEGORIES as c, ubbt_FORUMS as f WHERE (f.FORUM_IS_ACTIVE = 1) AND (f.CATEGORY_ID=c.CATEGORY_ID) GROUP BY c.CATEGORY_ID ORDER BY c.CATEGORY_SORT_ORDER


Google search says it's a permission problem. Everything else seems to be working fine - I can still post. Any idea what the problem is?