On backups, get error reading response length from fastrpc.cgi

SYSTEM INFORMATION
OS type and version Ubuntu 20.04
Webmin version REQUIRED
Virtualmin version REQUIRED
Related packages SUGGESTED

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.

@Beany,

Could you provide a screenshot with the error?

No help on this?
Playing havoc with automated backups.

Any out-of-memory errors or timeouts in the logs on the receiving sever?

SSH or Rclone might be a workaround.

Richard

Have you opened all the necessary ports for FastRPC between the sending and receiving servers? (10000-10100)

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.