Cannot register user - collation error

PunBB 1.2.12 / Linux server / MySQL 4.1.10a-log / PHP 4.3.9

I get this error when trying to register a user:

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Error: Unable to fetch user info.

Illegal mix of collations (ascii_general_ci,IMPLICIT) and (latin1_swedish_ci,COERCIBLE) for operation '=' (Errno: 1267)

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I do not have access to the servers.

I tried setting 'lang_encoding' to utf-8, but that did not help. I left it so and then added '$db->query("SET NAMES 'UTF8'");' at end of include/dblayer/common_db.php and an ini_set to config.php to make utf8 default character set. But these didn't help either.

Finally, I reverted all those changes except for the lang_encoding. Then I altered the character-set and collation of the PunBB database to ascii and ascii_general_ci (and also of punbb_users table). But this also did not help.

Please help.