Ernesto,
with a Linux server running SuSE 7.2, a 2.4.x kernel and pppoe, I
experienced EXACTLY the same problem; total (and reproduceable) server
hangs after specific actions of some users and an overall unstable
machine behaviour. A real pain in the ass.
I nearly dissected the whole machine, configured extensive logging,
traced the kernel back and forth, and finally came to the conclusion
that pre2.4.10 kernels and pppoe, together with certain user requests on
the network level, aren't a very good combination. IMO, the older, buggy
2.4.x kernels should not be used on production systems, they have some
more or less serious flaws in diverse sub systems, such as the IP stack;
the lowest 2.4.x kernel version I'd dare to use would be 2.4.14. Go
figure.
After changing the kernel to 2.2.20, kicking SuSE's pppoe/pppox
implementation and a manual configuration of Roaring Penguin's pppoe (
http://www.roaringpenguin.com/pppoe/ ), the problem silently went away.
The system now runs smoothly, without any hangs or crashes.
Perhaps this helps.
Boris