Thread:Project:Support desk/Error creating thumbnail: Unable to save thumbnail to destination/reply (8)

I just ran across this problem as well while upgrading to 1.20.5. But I had this set in LocalSettings.php:

$wgTmpDirectory = "{$wgUploadDirectory}/tmp";

I solved it by commenting this line out completely. I assume I was trying to get around some prior bug. But now mediawiki finds the tmp directory without a problem. So my solution was to remove this (old) setting.