Migration from Zimbra Errors

I am following these instructions for migrating:
https://community.zextras.com/how-to-use-script-to-migrate-zimbra-to-carbonio-carbonio-ce/

When following step 7 for importing to the new environment, I'm getting a lot of these errors:

ERROR: service.FAILURE (system failure: POST failed, status=504 504)

How do I begin to hunt them down?

I'm also getting a few of these:

ERROR: zclient.IO_ERROR (Read timed out) (cause: java.net.SocketTimeoutException Read timed out)

I had already increased the timeout, so, um, increase it more?