Another attempt produced on half of the machines in the model: 

machine-8: 21:48:10 INFO juju.worker.migrationminion migration phase is now: VALIDATION
machine-8: 21:48:10 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:10 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:10 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:11 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:11 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:11 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:11 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:12 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:12 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:13 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:13 ERROR juju.worker.migrationminion validation failed: failed to open API to target controller: try again (try again)
machine-8: 21:48:13 INFO juju.worker.migrationminion migration phase is now: VALIDATION
machine-8: 21:48:13 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:14 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:14 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:14 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:14 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:14 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:15 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:15 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:16 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:17 INFO juju.api connection established to "wss://10.74.147.148:17070/model/0c3ae1d9-d660-4841-8330-423dd59d7c87/api"
machine-8: 21:48:17 ERROR juju.worker.migrationminion validation failed: failed to open API to target controller: invalid entity name or password (unauthorized access)
machine-8: 21:48:17 INFO juju.worker.migrationminion migration phase is now: ABORT
machine-8: 21:48:28 INFO juju.worker.migrationminion migration phase is now: ABORTDONE