Error after upgrading from 1.8 to 1.12 (no replies)

Warning: htmlspecialchars() [<a href='function.htmlspecialchars'>function.htmlspecialchars</a>]: Invalid multibyte sequence in argument in /var/www/html/piwik/core/DataTable/Filter/SafeDecodeLabel.php on line 52

Backtrace -->

#0 Piwik_ErrorHandler(...) called at [:]
#1 htmlspecialchars(...) called at [/var/www/html/piwik/core/DataTable/Filter/SafeDecodeLabel.php:52]
#2 Piwik_DataTable_Filter_SafeDecodeLabel::safeDecodeLabel(...) called at [/var/www/html/piwik/core/DataTable/Filter/SafeDecodeLabel.php:67]
#3 Piwik_DataTable_Filter_SafeDecodeLabel->filter(...) called at [/var/www/html/piwik/core/DataTable.php:390]
#4 Piwik_DataTable->filter(...) called at [/var/www/html/piwik/core/DataTable.php:415]
#5 Piwik_DataTable->applyQueuedFilters(...) called at [/var/www/html/piwik/core/API/ResponseBuilder.php:281]
#6 Piwik_API_ResponseBuilder->handleDataTable(...) called at [/var/www/html/piwik/core/API/ResponseBuilder.php:77]
#7 Piwik_API_ResponseBuilder->getResponse(...) called at [/var/www/html/piwik/core/API/Request.php:147]
#8 Piwik_API_Request->process(...) called at [/var/www/html/piwik/core/ViewDataTable.php:423]
#9 Piwik_ViewDataTable->loadDataTableFromAPI(...) called at [/var/www/html/piwik/core/ViewDataTable/HtmlTable.php:79]
#10 Piwik_ViewDataTable_HtmlTable->main(...) called at [/var/www/html/piwik/core/Controller.php:147]
#11 Piwik_Controller->renderView(...) called at [/var/www/html/piwik/plugins/Referers/Controller.php:247]
#12 Piwik_Referers_Controller->getKeywords(...) called at [/var/www/html/piwik/plugins/Referers/Controller.php:125]
#13 Piwik_Referers_Controller->getSearchEnginesAndKeywords(...) called at [:]
#14 call_user_func_array(...) called at [/var/www/html/piwik/core/FrontController.php:125]
#15 Piwik_FrontController->dispatch(...) called at [/var/www/html/piwik/index.php:47]