I have clean install of eZ publish plain_site (3.9.3, 3.10.0, 4.0.0) with unicode support and selected two languages in setup: rus-RU (as main), eng-GB.
When I have try to search any cyrillic text twice in search module, second attempt crashes eZ with following error message:

Fatal error: A database transaction in eZ Publish failed
...
Error: eZMySQLDB Mar 10 2008 22:37:46 
Query error: Duplicate entry '' for key 2. Query: INSERT INTO
                              ezsearch_search_phrase ( phrase, phrase_count, result_count) VALUES ( '񂰵񃱂', 1, 0 )
...

String value for 'phrase' column in sql insert command contains unrecognized symbols (show as squares).
Everything works perfectly when searching text contains only latin chars.

Steps to reproduce

I have created fix for this bug
kernel\classes\ezsearchlog.php - line 47:

...
        $phrase = strtolower( trim( $phrase ) );
...

replacing "strtolower" call with "mb_strtolower" fixes this issue