I have this error log in my ACP that appears every few hours every day. Haven't isolated what causes it yet and it doesn't seem to be causing any obvious issues until now. When the error happens, the site goes down and displays the error below for about 5 minutes before going back up by itself.
Is it happening at a particular time? If so, there's likely a backup process going on. Unfortunately, this appears to be internal to MySQL so there's not much we can do within XenForo. Your host may be able to increase the MySQL table_definition_cache and/or table_open_cache values to help resolve the issue.