Error after upgrading to 1.7

Hi. I´ve just upgraded to CB 1.7 (running Joomla 1.5.24) and when I ran the check user´s database i got the following error:

ERROR: sql query: SELECT u.id FROM #__users u LEFT JOIN #__core_acl_aro a ON a.section_value = 'users' AND a.value = CAST( u.id AS CHAR ) WHERE a.value IS NULL : returned error: Illegal mix of collations (utf8_unicode_ci,IMPLICIT) and (utf8_general_ci,IMPLICIT) for operation '=' SQL=SELECT u.id FROM jos_users u LEFT JOIN jos_core_acl_aro a ON a.section_value = 'users' AND a.value = CAST( u.id AS CHAR ) WHERE a.value IS NULL

Any idea of what It could be?

Thanks in advance