Thanks Josh! Do you think the assumed cause for these login problems was right?

Doing a

SELECT COUNT(*)
FROM w3t_Users
WHERE U_Language = 'english'

gives 2332 results,
<> 'english' gives 511 results,
<> '' gives 2843 results,
on a board with 55988 members. Isn't that odd?

Based on this observation, isn't this a better query;

UPDATE w3t_Users
SET U_Language = 'english'
WHERE U_Language <> 'english'