[c-nsp] Weird problem with 2960S and desktop switch

[Garry]

Hi,

I've run into a strange problem at a customer that I can't really get a
grip on ...

At the customer site (remote), there's a stack of two 2960S switches,
running a flat setup (no VLANs for customer pointing ports), so
everything is access VLAN 1.
At some areas of the customer location, not enough ethernet ports were
available, so they set up a desktop switch to hook up additionals PCs or
phones. Anyway, when they did, the port went down as err-disabled due to
BPDU guard. Suspecting an accidental loop, I had the customer disconnect
all downlink ports from the desktop switch, thereby removing and chance
of having a loop. After shut/no shut, the port again went down due to BPDU.
Disabling BPDU guard caused the whole switch to lock up while the
desktop switch was connected, even though checking the logs after
removing the desktop switch (and a bit of additional waiting time, in
which I assume spanning tree came back up) did not show any errors or
messages between the int up and down of that port.

What am I missing here?

Tnx, -gg