This started happening to me out of the blue since yesterday. I am not using IPv6 with any of my subnets, however even with network discovery being off, the IPv6 address on the Rack Controllers (I have two) was added under fabric-0 and this was causing the Region Controllers (Two again on separate machines) go completely haywire! 

The initial workaround was to configure the untagged subnet to only use one Rack Controller for DHCP services and then I went ahead and deleted the IPv6 auto-added subnet from fabric-0. 

After that re-added the second Rack controller to restore HA and everything seems to be normal so far.

The trace looks the same as what was originally reported:

2024-02-19 03:16:46 maasserver.dhcp: [info] Successfully configured DHCPv4 on rack controller 'maas-rack-2 (6ks83s)'.
2024-02-19 03:16:46 maasserver.rack_controller: [critical] Failed configuring DHCP on rack controller 'id:2'.
	Traceback (most recent call last):
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1750, in gotResult
	    current_context.run(_inlineCallbacks, r, gen, status)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1740, in _inlineCallbacks
	    status.deferred.errback()
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 700, in errback
	    self._startRunCallbacks(fail)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 763, in _startRunCallbacks
	    self._runCallbacks()
	--- <exception caught here> ---
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 857, in _runCallbacks
	    current.result = callback(  # type: ignore[misc]
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/rack_controller.py", line 281, in <lambda>
	    d.addErrback(lambda f: f.trap(NoConnectionsAvailable))
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/failure.py", line 451, in trap
	    self.raiseException()
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/failure.py", line 475, in raiseException
	    raise self.value.with_traceback(self.tb)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 857, in _runCallbacks
	    current.result = callback(  # type: ignore[misc]
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/rack_controller.py", line 300, in unwatch_if_does_not_exist
	    f.trap(RackController.DoesNotExist)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/failure.py", line 451, in trap
	    self.raiseException()
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/failure.py", line 475, in raiseException
	    raise self.value.with_traceback(self.tb)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1656, in _inlineCallbacks
	    result = current_context.run(
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/failure.py", line 489, in throwExceptionIntoGenerator
	    return g.throw(self.type, self.value, self.tb)
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/dhcp.py", line 867, in configure_dhcp
	    config = yield deferToDatabase(get_dhcp_configuration, rack_controller)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 244, in inContext
	    result = inContext.theWork()  # type: ignore[attr-defined]
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 260, in <lambda>
	    inContext.theWork = lambda: context.call(  # type: ignore[attr-defined]
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/context.py", line 117, in callWithContext
	    return self.currentContext().callWithContext(ctx, func, *args, **kw)
	  File "/snap/maas/33524/usr/lib/python3/dist-packages/twisted/python/context.py", line 82, in callWithContext
	    return func(*args, **kw)
	  File "/snap/maas/33524/lib/python3.10/site-packages/provisioningserver/utils/twisted.py", line 857, in callInContext
	    return func(*args, **kwargs)
	  File "/snap/maas/33524/lib/python3.10/site-packages/provisioningserver/utils/twisted.py", line 203, in wrapper
	    result = func(*args, **kwargs)
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/utils/orm.py", line 771, in call_within_transaction
	    return func_outside_txn(*args, **kwargs)
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/utils/orm.py", line 574, in retrier
	    return func(*args, **kwargs)
	  File "/usr/lib/python3.10/contextlib.py", line 79, in inner
	    return func(*args, **kwds)
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/dhcp.py", line 786, in get_dhcp_configuration
	    config = get_dhcp_configure_for(
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/dhcp.py", line 639, in get_dhcp_configure_for
	    peer_name, peer_config, peer_rack = make_failover_peer_config(
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/dhcp.py", line 508, in make_failover_peer_config
	    interface_ip_address = get_ip_address_for_rack_controller(
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/dhcp.py", line 258, in get_ip_address_for_rack_controller
	    return get_ip_address_for_interface(interface, vlan, ip_version)
	  File "/snap/maas/33524/lib/python3.10/site-packages/maasserver/dhcp.py", line 232, in get_ip_address_for_interface
	    for ip_address in interface.ip_addresses.all():
	builtins.AttributeError: 'NoneType' object has no attribute 'ip_addresses'