On certain occasions, and if you’re running Plesk on a dedicated server (like I do), there are certain problems with file access and other upload permissions which aren’t allowed unless you turn file permission to 777!
Obviously this isn’t the recommended way to operate a website, turning directories and files all open with rwx is kind of risky.
So I have found a solution that works everytime, simply change the “PHP Support” to run as “FastCGI Application” instead of the default “Apache”. This option is found under the Hosting setting of the domain.
I must point out that this hack only works if you’re running Plesk (10+) on Linux Servers.