New mailbox module uses GREATEST() function which is not available in 5.0 Mysql.
[Tue Aug 26 19:19:10 2014] [ow_core_log] [Exception] Message: SQLSTATE[42000]: Syntax error or access violation: 1305 FUNCTION kin_17.GREATEST does not exist File:/home/admin/v17.kin.om/ow_core/database.php Line:707 Trace: [#0 /home/admin/v17.kin.om/ow_core/database.php(707): PDOStatement->execute() | #1/home/admin/v17.kin.om/ow_core/database.php(398): OW_Database->execute(' SELECT `conv`....', Array) | #2 /home/admin/v17.kin.om/ow_plugins/mailbox/bol/conversation_dao.php(411):OW_Database->queryForList(' SELECT `conv`....', Array) | #3 /home/admin/v17.kin.om/ow_plugins/mailbox/bol/conversation_service.php(2741):MAILBOX_BOL_ConversationDao->findConversationItemListByUserId(1598, Array, 0, 10) | #4 /home/admin/v17.kin.om/ow_plugins/mailbox/bol/conversation_service.php(2192):MAILBOX_BOL_ConversationService->getConversationListByUserId(1598, 0, 10) | #5 /home/admin/v17.kin.om/ow_plugins/mailbox/bol/conversation_service.php(2110):MAILBOX_BOL_ConversationService->getLastDataAlt(Array) | #6 /home/admin/v17.kin.om/ow_plugins/mailbox/classes/event_handler.php(872): MAILBOX_BOL_ConversationService->getLastData(Array) | #7/home/admin/v17.kin.om/ow_core/event_manager.php(200): MAILBOX_CLASS_EventHandler->onPing(Object(OW_Event)) | #8 /home/admin/v17.kin.om/ow_system_plugins/base/controllers/ping.php(20):OW_EventManager->trigger(Array, Object(OW_Event)) | #9 /home/admin/v17.kin.om/ow_core/request_handler.php(263): BASE_CTRL_Ping->index(Object(OW_Event)) | #10/home/admin/v17.kin.om/ow_core/application.php(327): OW_RequestHandler->dispatch(Array) | #11 /home/admin/v17.kin.om/index.php(73): OW_Application->handleRequest(Array, Array) | #12{main}]
So please update requirements or downgrade the code.