WP-AdServe Problem

Hi,
I installed WP-AdServer and works at the back but it does not print the banners. I double checked tags and most frequent problems posted in forums but still not working. These are the error form the SQL.

[Tue May 06 12:35:06 2008] [error] [client 194.60.38.198] WordPress
database error Illegal mix of collations (latin1_swedish_ci,IMPLICIT)
and (utf8_general_ci,COERCIBLE) for operation 'concat' for query SELECT
count(id) FROM wp_adserve WHERE ((credits = -1) or (credits > 0)) AND
(active = 1) AND (concat(keywords,' ') LIKE '%ad_big_2 %'); made by
iri_AdServe_GetBanner, referer...

Could someone help me with this matter?
Thanks,