I have finally had time to upgrade my older box to 2.6.18.1. After
booting I got a message about disabling IRQ #15 making it impossible to
access my harddrive connected to the secondary IDE port.

I tried different kernelversions. The problem started with 2.6.18-rc5
(rc4 was the last one which was ok).

Starting with the irqpoll option made the drive work again, but I have
still a disabling IRQ message in the log.
Looking into /proc/interrupt shows over 90000 IRQs for interruptline 15
directly after booting.

Attached a dmesg and an output of lspci.