MyBB SQL Error

MyBB has experienced an internal SQL error and cannot continue.

SQL Error:
3 - Error writing file '/tmp/MYqCGMNS' (Errcode: 28 - No space left on device)
Query:
SELECT u.*, f.* FROM mybb_users u LEFT JOIN mybb_userfields f ON (f.ufid=u.uid) WHERE 1=1 ORDER BY u.postnum DESC LIMIT 21900, 20

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