Thread:Project:Support desk/Problem uploading files: Could not create directory "mwstore://local-backend/local-public/5/54"./reply (21)

I think the following will be useful for that type of error.

If you use Vagrant and your directory is a Shared Folder then Vagrant will reset the access rights every time you try to change it. In this case you have to use  in your Vagrantfile:

config.vm.synced_folder "../www", "/var/www/html", mount_options: ["dmode=777", "fmode=777"]

this way your www directory will have 777 rights and the web server will be able to write to it. See also related stackoverflow question.