29.12.2015 18:22, Sowmini Varadhan пишет:

Do you have admin control over the ubuntu router?
If yes, you might want to check the shared_media [#] setting 
on that router for the interfaces with overlapping subnets.
(it is on by default, I would try turning it off).

Updated testing results:
After I disabled shared_media not only for "all", but
also for _all_ interfaces individually, the problem seem to
have stopped. So thanks for these hints.
Now, as the media is actually really shared (same NIC/cable),
I just wonder what's going on here.

And unfortunately we still don't know why these redirects are
ever accepted...