Backup Fetch Failed

Hello Community,

I can't backup my getgrav installation. I am using v1.58.
This ist the message which pops up on the right side (with a red border) on the dhasboard when I press "Backup".

Fetch Failed:
html, body, #partner, iframe {
            height:100%;
            width:100%;
            margin:0;
            padding:0;
            border:0;
            outline:0;
            font-size:100%;
            vertical-align:baseline;
            background:transparent;
        }
        body {
            overflow:hidden;
        }

        document.write(
                '

Can anybody help me to resolve this issue?

Edit:
This might be the problem:

I have tried to set permissions of cache and Backup Folder to 755.

I saw that a file with the name "kalbitzer-music-20190319104940.zip.2kWxPv" is generated in the Backup Folder, but in the Admin panel I still get the error message.

What's the "2kWxPv" behind the ".zip"?

This is how my backup-folder looks like.
Does anyone know how it get zips from these files?

I can't unzip them when I rename them to .zip