Comment 16 for bug 1915511

The logs from the failed unit in comment #13 confirm the problem.

The issue is as per https://bugs.launchpad.net/juju/+bug/1882827 which we discussed earlier.
This can be seen looking at this log line

RROR juju.worker.migrationminion worker.go:197 validation failed: failed to open API to target controller: try again (try again)

The "try again" error is what happens when an agent connection gets rejected because rate limiting has been applied by the controller.

As explained in the bug, you can adjust the agent-ratelimit-max and agent-ratelimit-rate controller config.

Also, as suggested earlier, you should also upgrade the source controller to 2.8.8. The above bug has a fix which landed in 2.8 which makes the agents a little more tolerant to a busy controller during the validation phase.