Please help regarding file upload
I too am looking for this folder.
user@server:~$ sudo find / -samefile /local-backend
find: `/local-backend': No such file or directory
local-backend isn't the name of a folder. Its a stand in mediawiki uses for whereever you have configured for the file uploads to go. In most installs, the folder "local-backend/local-public" refers to the images subdirectory of your MediaWiki install.
I figured out what caused it on my end. I was using importImages.php to batch upload a bunch of images. I did this as root, so all the sub-directories in $IP/images that were created during the batch upload were owned by root instead of www-data. When the web interface, which uses www-data user, tried to create a subdirectory it didn't have write permissions. I recursively changed the ownership of all the directories back to www-data and it worked. Here's the command.
$ chown -R www-data images
This didn't change the group ownership, but that shouldn't matter.