When we do server backups, we seem to be getting this error:
… upload failed! Error reading response length from fastrpc.cgi
It seems to happen only on backups where the backup file is reasonably large.
For example, we can do an incremental backup where it works fine, but if we switch to full backup (using identical settings) we get this error. It also happens when we do a large incremental backup.
All required firewall ports are open, no timeout errors reported in the logs.
We eventually decided to do a local backup, then rsync the backup file to a remote backup server.
Not as clean as the original solution but at least it works.