I ran unto an issue when upgrading from 1.24.0 to 1.27.0.
Since I'm on a shared server and have multiple Mediawiki installations, the system temp folder is being shared across them all. This is causing a MWException error on page load. Obviously this will cause problems since temp content of one wiki will overwrite the content of another.
So I've declared a custom temp directory: (Paste into LocalSettings.php)
$wgTmpDirectory = "$IP/images/temp";
While this fixes the issue, I'm not sure it's a secure practice. I see many other people using this fix... But is it safe to have the temp folder PUBLICLY visible?