Multi Upload fails after one successful file upload.

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I struggle a bit with this:

When uploading multiple files, then the first file always gets trough but all other files not. They throw a: "The file was only partially uploaded" Error ($_FILES Error -> UPLOAD_ERR_PARTIAL).

Some more facts to that:

> seems to happen since a Plesk Update on the server, all domains suffer equally from it. It worked on all of them before and not anymore now, no matter what concrete5 version they use.

> Does only happen with the concrete5 SWF file uploader. A html 5 file uploader can upload files without any problem, any size.

> If the files that are very very small (eg. less then 40kb) they can be uploaded. Remember that the first file always gets uploaded, no matter the file size

> Happens in all browsers, no matter the version or type

> It has nothing to do with the file name (tried easy names like 1.jpg, 2.jpg, etc...)

I think it's maybe a config on the server (Unix), but the guys who manage our server tried already a lot and can't figure it out. Any idea what it could be or ever heard of something like this?