The one time I had that issue I was making a web-site for a client that insisted on using CentOS and Plesk. After many searches and what seemed like thousands of articles I found it was a file permissions issue. At first that seems simple to fix, right? Nope, it isn’t caused by security permissions eg; 755, 644. It tuned out that Plesk uses a “tweaked” version of PHP form some reason unknown to me.
I spent entirely too much time on the issue. If I remember correctly, the FTP or server files were owned by Apache which is the cause of the problem. I read several posts where others claimed to have resolved the problem with different hacks and tweaks. I’m capable of following directions, the solutions simply didn’t work for my situation.
The owners of the site (the same people that insisted on using the services) were getting impatient, and I spent twice the normal time on the project and still had issues. I eventually set the file permissions to 777 to buy me enough time to convince them it would continue to be a pain.
We switched to a host that provided CPanel not Plesk and it was business as usual.