After adding the reg key;
The backup is uploaded correctly!
So we will try it on another server to confirm this fix (for us).
I've open the Windows registry path "HKEY_LOCAL_MACHINE\SOFTWARE\PLESK\PSA Config\Config"
and created the "FtpRequestTimeOut" DWORD key with a value of "60000" seconds (about 16,6 hours).
The backup is uploaded correctly!
So we will try it on another server to confirm this fix (for us).