Sorry for wasting everyone's time - it WAS a network cabling problem.
The little caps on the terminating resistors at BOTH ends had come loose.
A few days ago I asked a question about networking with the pl12 kernel.
From the news I understand that I'm not the only one with problems. I tried
to use the latest SLS 1.03 release which includes the latest (final) pl12
release of the kernel.
I received all kind of hints of how to organize my /etc/rc.net
startup-file(s), but none of these worked and my routing remained broken.
There are some suggestions left that I didn't try (yet):
* Using the 4.4.2 library (Linus), which is however still in the alpha
phase (see comp.os.linux/53589).
* Using a different /etc/route (Jon Tombs in comp.os.linux/53653).
Right now I installed the Slackware 1.01 release (from nic.funet.fi), which
still includes the earlier version of the kernel (pl12 ALPHA of Aug 7 if I'm
correct). With this package my networking worked without any major probles.
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