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Two machines, say, old and new, are in the same local network and connected via WiFi to the same router (Speedport). Both old and new run Debian stable, have openssh-client and openssh-server (but not nscd) installed, and have a local user user. As expected, we have

user@old$ ping4 -vv new
PING new.Speedport… (192.168.2.100) 56(84) bytes of data
64 bytes from new (192.168.2.100): icmp_seq=1 ttl=64 time=178 ms
…
user@old$ ping6 -vv new
PING new(new (fe80::34bc:bf13:5e2b:69c0) 56 data bytes
64 bytes from new (fe80::34bc:bf13:5e2b:69c0%wlp3s0): icmp_seq=1 ttl=255 time=8.45 ms
…

However, often (e.g., after the restart of both machines):

user@new$ ping4 -vv old
ping4: old: Address family for hostname not supported
user@new$ ping6 -vv old
PING old(old (fe80::396:d3c8:f616:b376)) 56 data bytes
From new (fe80::34bc:bf13:5e2b:69c0%wlp179s0) icmp_seq=1 Destination unreachable: Address unreachable
…

Moreover, I can ssh from old to new but often (e.g., after the restart of both machines) not from new to old (ssh -vv says “connect to address ‹an IPv6 address› port 22: Invalid argument”). Typing in the actual IPv4 address 192.168.2.107 of old instead of old verbatim works like a charm for starting both ssh and ping4 on new.

What to do? How do I further debug or even repair the failures of ssh old and ping old (when started on new)?

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