user watches don't work after an upgrade from 2.x to 3.0

• Error

After an upgrade from 2.x to 3.0 if I edit a page while user watches are enabled I get an error similar to this :

Errore

An error occured in a database query!

Context:
File tiki-editpage.php
Url tiki-editpage.php?page=Italiano
Query:
select tuw.*, tup1.`value` as language, tup2.`value` as mailCharset from `tiki_user_watches` tuw left join `tiki_user_preferences` tup1 on (tup1.`user`=tuw.`user` and tup1.`prefName`='language') left join `tiki_user_preferences` tup2 on (tup2.`user`=tuw.`user` and tup2.`prefName`='mailCharset') where `event`=?and (`object`=?) UNION DISTINCT select tgw.watchId, uu.login, tgw.event, tgw.object, tgw.title, tgw.type, tgw.url, uu.email, tup1.value as language, tup2.value as mailCharset from tiki_group_watches tgw inner join users_usergroups ug on tgw.`group` = ug.groupName inner join users_users uu on ug.userId = uu.userId and uu.email is not null and uu.email <> '' left join `tiki_user_preferences` tup1 on (tup1.`user`=uu.`login` and tup1.`prefName`='language') left join `tiki_user_preferences` tup2 on (tup2.`user`=uu.`login` and tup2.`prefName`='mailCharset') where `event`=?and (`object`=?)
Values:
0 wiki_page_changed
1 Italiano
2 wiki_page_changed
3 Italiano
Message:
Built query was probably:
select tuw.*, tup1.`value` as language, tup2.`value` as mailCharset from `tiki_user_watches` tuw left join `tiki_user_preferences` tup1 on (tup1.`user`=tuw.`user` and tup1.`prefName`='language') left join `tiki_user_preferences` tup2 on (tup2.`user`=tuw.`user` and tup2.`prefName`='mailCharset') where `event`='wiki_page_changed'and (`object`='Italiano') UNION DISTINCT select tgw.watchId, uu.login, tgw.event, tgw.object, tgw.title, tgw.type, tgw.url, uu.email, tup1.value as language, tup2.value as mailCharset from tiki_group_watches tgw inner join users_usergroups ug on tgw.`group` = ug.groupName inner join users_users uu on ug.userId = uu.userId and uu.email is not null and uu.email <> '' left join `tiki_user_preferences` tup1 on (tup1.`user`=uu.`login` and tup1.`prefName`='language') left join `tiki_user_preferences` tup2 on (tup2.`user`=uu.`login` and tup2.`prefName`='mailCharset') where `event`='wiki_page_changed'and (`object`='Italiano')

Only if I disable the feature "monitoring" the error doesn't show up.

It was a problem with some tables in my database, infact if I query directly the database I get the error :
Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation '='

I checked my db and I found some tables that was set in utf8_general_ci while the others (almost all) was set to utf8_unicode_ci

To solve my issue I had to convert the tables that has a different collation to the right collation :
ALTER TABLE tiki_webservice_template CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci

This bug has been demonstrated on show2.tiki.org

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This bug has been demonstrated on show.tikiwiki.org

Please demonstrate your bug on show.tikiwiki.org