In my installation, showing Special:ContributionScores throws the following error:
Error 1055: Expression #4 of SELECT list is not in GROUP BY clause and contains nonaggregated column 's.page_count' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
As a temporary workaround I can use:
$wgSQLMode="";
in LocalSettings.php.
The problem can probably be fixed changing line 188:
'GROUP BY' => 'user_name',
to
'GROUP BY' => 'user_name, page_count, rev_count',