`zfs send` immediately stops with "Assertion failed: (progress)"

[Lapo Luchini]

I migrated this server a few times already on new hardware over the 
years, by using `zfs send`.
Even right now I'm using `zrepl` to periodically have a remote copy to a 
distant site (for disaster recovery).
But trying to migrate on a new server in the same server farm recently 
I'm seeing this error:

$ sudo zfs send -Rv z at recentSnapshot >/dev/null
Assertion failed: (progress), file 
/usr/src/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_sendrecv.c, 
line 1511.
This is happening in a freshly updated FreeBSD 12.2-RELEASE-p2.

I don't know how to reproduce it (or debug it) and the server is serving 
production data, so I can only do non-intrusive tests.

I guess it has something to do with clones, given the comments at line 
1480 and line 1492 so maybe I can print something with `zdb`?
Any idea how to debug (and solve)?

PS: it's a bit out of topic, but since the code in latest OpenZFS seems 
to be exactly the same, I opened a bug there too:
https://github.com/openzfs/zfs/issues/11457

-- 
Lapo Luchini - http://lapo.it/