replication does not work after upgrade to 12.0-U2.1

Upgraded from 11.2-U8 to 12.0-U2.1 (with a stopover at 11.3-U5).  Manual upgrade, since there was option to upgrade to 12.0 directly from 11.2. or 11.3 for that matter.

Replication before upgrade to a 12.0-U2.1 system worked fine before upgrade.  After upgrade, and after fixing ssh connections, replication still does not work.

Turned on DEBUG logging in Replication Task, I got some weird output in debug.log:

[2021/03/02 19:55:05] INFO     [replication_task__task_3] [zettarepl.replication.run] For replication task 'task_3': doing push from 'vol' to 'vol' of snapshot='auto-20210302.1800-2d' incremental_base='auto-20210302.1040-2d' receive_resume_token=None encryption=False

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.base_ssh.root@vis-backup.shell.95.async_exec.5028] Running ['zfs', 'umount', 'vol']

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.paramiko.replication_task__task_3] [chan 60] Max packet in: 32768 bytes

[2021/03/02 19:55:05] DEBUG    [Thread-194] [zettarepl.paramiko.replication_task__task_3] [chan 60] Max packet out: 32768 bytes

[2021/03/02 19:55:05] DEBUG    [Thread-194] [zettarepl.paramiko.replication_task__task_3] Secsh channel 60 opened.

[2021/03/02 19:55:05] DEBUG    [Thread-194] [zettarepl.paramiko.replication_task__task_3] [chan 60] Sesch channel 60 request ok

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.base_ssh.root@vis-backup.shell.95.async_exec.5028] Reading stdout

[2021/03/02 19:55:05] DEBUG    [Thread-194] [zettarepl.paramiko.replication_task__task_3] [chan 60] EOF received (60)

[2021/03/02 19:55:05] DEBUG    [Thread-194] [zettarepl.paramiko.replication_task__task_3] [chan 60] EOF sent (60)

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.base_ssh.root@vis-backup.shell.95.async_exec.5028] Waiting for exit status

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.base_ssh.root@vis-backup.shell.95.async_exec.5028] Error 1: "cannot unmount 'vol': not currently mounted\n"

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.local.shell.1.async_exec.5029] Running ['zfs', 'send', '-V']

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.local.shell.1.async_exec.5029] Error 2: 'missing snapshot argument\nusag.... list, run: zfs allow|unallow\n'

[2021/03/02 19:55:05] DEBUG    [replication_task__task_3] [zettarepl.transport.local.shell.1.async_exec.5031] Running ['sh', '-c', 'exec 3>&1; eval $(exec 4>&1 >&....] && exit $pipestatus1; exit 0']

[2021/03/02 19:55:06] DEBUG    [replication_task__task_3.async_exec_tee.wait] [zettarepl.transport.local.shell.1.async_exec.5031] Error 141: None

[2021/03/02 19:55:06] DEBUG    [replication_task__task_3.process] [zettarepl.transport.local.shell.1.async_exec.5030] Error 141: 'No ECDSA host key is known for....Host key verification failed.\n'

[2021/03/02 19:55:06] DEBUG    [replication_task__task_3.monitor] [zettarepl.transport.local.shell.1.async_exec.5031] Stopping

[2021/03/02 19:55:06] WARNING  [replication_task__task_3] [zettarepl.replication.run] For task 'task_3' at attempt 1 recoverable replication error RecoverableReplicationError('Broken pipe (No ECDSA host key is known for vis-backup.an.intel.com and you have requested str

ict checking.\nHost key verification failed.\n)')

[2021/03/02 19:55:06] ERROR    [replication_task__task_3] [zettarepl.replication.run] Failed replication task 'task_3' after 1 retries

[2021/03/02 19:55:06] DEBUG    [Thread-194] [zettarepl.paramiko.replication_task__task_3] EOF in transport thread